My Stepsister is My Ex-Girlfriend (WN) Chapter 1

The Ex-Couple Screams – “Damn you, God!”



I was standing at the entranceway of my house playing out a staring contest like a hoodlum.

My opponent was a girl of the same age as me. There was nothing more or nothing less between us—or so I wanted to say, but there was actually something between us, or perhaps I should say used to be.

“…………Where do you think you are going, Mizuto.”

“…………I should be asking you that, where are you going, Yume.”

The girl spoke, then me, followed by silence.

That was now the third time.

In fact, I already knew where this girl was heading to without even the need to ask. Her destination was the large bookshop in front of the station. Today was the day when a certain mystery novel publisher released their new books. I was off to get one of the new series from that same publisher and it appears that this girl had the same idea.

That was why we were probably going to end up going through this entranceway together, walk side-by-side to the bookshop, head towards the same corner of it, and line up before the register in succession.

That way, wouldn’t it totally look like we’re a couple sharing the same interest in fiction.

And that was precisely what either of us wanted to prevent absolutely.

In conclusion, we were stuck in a deadlock.

The timing of that either of us leave the house must be different, but as for the problem of which of us is leaving first—And in order to decide that, here we are now trading diversionary tactics.

The problem would be solved if we talked this out, you say?

No way is that happening. There was no a single thing in the world which could be solved through conversation with this girl.

Not to mention—

“—Hmm? Yume and Mizuto? What are you guys doing there?”

Ms. Yuni came over from the living room dressed in a suit.

She was none other than the person who became my mother just one week ago.

In other words, she was the partner of my father’s remarriage, and—the actual mother of this girl before me.

“Aren’t the two of you going out?”

“I’m just about to go.”

‘Excuse me’, or so I was going to transition into smoothly so as to use this opportunity to make the first move, but Ms. Yuni interjected before I could.

“Oh, could you be heading for the bookshop at the front of the station? I’ve heard that you are also a book-lover! In that case, I guess you’re headed the same way as Yume then? After all, all this girl goes is to either the bookshop or the library whenever she says she’s going out.”


“Come on, Mom……”

“Ah! Could it be that the two of you are going together! I’m so glad, Mizuto! You sure are getting along well with Yume! Please continue to take care of her. Yume’s kinda shy after all,”


Being told as much, there was nothing I could do but nod in reply.

While at the same time, I could feel a gaze of death right next to me.

“Well now, I’m leaving first. Have a safe trip, the two of you! Be sure to play nice as siblings, okay!”

With that, Ms. Yume disappeared beyond the entrance.

Leaving behind me and her—us siblings.

Indeed, we were brothers and sisters.

That is, by law.

As fellow stepchildren—.

“……Why did you have to nod.”

“……It’s not like I have a choice, with the flow she going at.”

“Why is it that I have to get along with someone like you?”

“Who cares. You think I want to get along with you.”

“That’s the kind of passivity that I hate about you, damn otaku.”

“And that’s the kind of self-centeredness I hate about you, damn nerd.”

However, our parents never knew.

Only she and I knew the real relationship between the two of us.

That I, Irido Mizuto—

and her, Irido Yume—

—had been dating each other until about a mere two weeks ago.


In hindsight, with what could only be described as a youthful indiscretion, I had something which you could call a girlfriend from the second to the third year of middle school.

If you were to ask me what was our first meeting like, I could only think of that moment near the end of July, just as we entered the summer break. It was during early afternoon in the library—she was standing atop the stepping stool, reaching her hand out towards the topmost row on the bookshelf.

As it was such a clichéd situation, I’m sure you could already imagine what followed, but I’ll narrate nonetheless. I had then took that book for her in her stead.

If I could go back in time, I would’ve told my past self to just leave that girl alone.

Yet, the me unknowing of the future then caught a glance of the title of the book I retrieved, and ignorantly spoke to her.

—You like mystery novels?

I wasn’t specifically a fan of mystery novels or what, if anything, I was in the faction that read any and everything—be it pure literature, romance novels or light novels, I read them all without reserve. That was why I knew the title of that classic mystery novel.

Even though I knew of it, I wasn’t particularly a fan of it.

Regardless, perhaps you could call this the disposition of a bookworm, I unconsciously grew excited seeing someone else pick up a book that I had read before. It was something akin to bull getting agitated upon seeing a red cloth, or some kind of unrestrainable instinct, and probably a trap by God, I thought.

A trap set up by God.

In other words, it was fate.

Through that fated meeting, the two of us found kindred spirits in each other in what was like a rendezvous in the library during the holiday where there was practically no one else around. And by end of the summer break in late August, I received a confession from her.

And in this process.

For the first time in my life, I gained something which you could call a girlfriend.

And her name was Ayai Yume.

That is, at the time at least.

Coming back to now.

Without the need for me to spell it out, that was the preface to collapse.

In fact, the chances of a love confession by middle schoolers which did not end up as such were probably around five percent or lower—in reality, you don’t ever see any middle school couples who went on to spend the rest of their lives together.

However, to the two of us then, it had seem that we would be the one.

As both Ayai and I were the type who doesn’t stand out much in school, our relationship proceeded in a uneventful manner. We would act unrelated before others, while rendezvousing in a corner of the school library, or the bookstores with cafes or public library on weekends, sharing in our interest as our love blossomed.

Of course, we also did stuff that normal couples did.

Ayai basically did not spoke to anyone in her class, having stranger anxiety. This caused her to appear mature so to speak in a positive light, or gloomy if a negative light. Thus, we may have progress slower than most, but still we went on dates, held hands, and kissed each other awkwardly—there was nothing particularly of note about us; we just followed the natural progression of any other lovers.

Our first kiss was on the splitting of paths back home from school bathed in the setting sun. I could still remember Ayai’s smile with slightly flushed cheeks after our kiss, which was more of a graze of our lips than a touch, as clear as a picture in my mind.

And the current me only has s single response to that snapshot.

Drop dead.

Be it that girl, or the me of that time.

……In any case, that was how we were steadily moving along our relationship, when the turning point slowly approached around the time we promoted to the third year of middle school.

The trigger was the improvement of Ayai’s stranger anxiety.

In the course of her relationship with me, her communication skills also improved—and she made numerous friends in her new class. Her progress was remarkable; no longer would she be without a partner during physical education, a common sight in her second year.

She herself was also tremendously happy about this improvement, and so did I as I congratulated her on the surface.

That’s right, only on the surface.

Then, what of on the inside—that would ended up as my regret. Even as I praised her for her growth, the feelings of wanting to hog her all for myself spreaded within me unconsciously.

The only one who knew of her cute points, her cheerful side, and her smile should have been me only—and so on.

But, that was wrong.

And I indirectly imbued my words with such feelings, causing Ayai to feel troubled as she tried to appease me without even knowing why. But that only got on my nerves even further.

Indeed—I had known. The underlying factor may have been the change in Ayai due to her growth, but the direct reason was because of my trivial desire to monopolize her. She had not done anything wrong. The one who was wrong in the first case was me, I admit it.


Notwithstanding that.

Let me raise a defense for myself. I had then realized my own foolishness at that time, and even apologized to her formally. I had become jealous due to such and such, and I’m sorry for taking it out on you. I’ll make it up to you, so please give me another chance—and so and so.

And then.

What do think that girl said?

—So you dislike me getting close to other people, but it’s okay for you to get close to other girls?


Who could blame me for replying with that.

According to her, I had cheated on her with another girl in the school library where we first met—that was news to me. I told her she probably misunderstood when I was speaking to some member of the library committee or something, but she kept insisting that that was cheating and wouldn’t listen to me at all.

In the end, I had to apologize one-sidedly.

Why the heck.

I admit that I was at fault for being too clingy. That’s why I apologized and lowered my head. It was up to her to decide if she wanted to forgive me. That I understood.

But why should I have to be admonished for what was a baseless misunderstanding on her part?

Okay, just hold up a moment, I know there’s times when the words exchanged get heated up due to impulsive feelings. After all, that’s what I did, so I apologized for it. And in that case, shouldn’t you do the same too? How can it be that I’m the only one to have to apologize for something unreasonable, while you don’t feel the slightest bit sorry? That’s just wrong.

—And with those feelings, we patched up on the surface and continued dating for a few months after that.

However—there was absolutely no way to repair two gears which no longer mashes with one another.

What had appeared to be appealing in the past was now a cause for irritation. We devolved to using sarcastic tones with each other, and it even became a pain to reply to each other’s message on the phone. The lack of reply further angered the other party and the gulf between us deepened.

We continued like this all the way to graduation only because we were both cowards.

It was solely due to the lack of courage on both sides.

Even so, at the point where there wasn’t even a single message or word exchanged on Valentine’s Day, the future was fixed—there was no further point to this. The two of us understood that implicitly.

I was the one to raise it, with graduation as the opportunity.

—Let’s break up.


It was over just like that. Not a single drop of tears flowed. She did not even appear angry, rather looking like she had been waiting all this time. I thought that I probably looked the same.

She was the girl which I had liked so much then, but now, she was nothing but an irreconcilable enemy to me, akin to what a viper is to a mongoose.

I must say, love is but a momentary misleading of the mind. I have finally freed myself from this beguilement—and with the feelings of letting off a heavy load, I graduated from middle school feeling light-hearted.

And it came till that night.

My father broached the topic this way.

—Mizuto, your father is thinking of getting remarried.


It seems that the beings known as humans do not stop fretting even at this age. I did pity Dad for raising a child by himself and did not particularly have any objections. I’d very much welcome a remarriage. Just proceed as you please? Not to mention, I had just completed the compulsory education.

I was feeling great. Which was why I had open-mindedly let slide what Dad said next.

—The other party also has a daughter…… Do you not mind?

Come on now, you’re telling me I’m gonna get a stepsister at this age? It’s as if I’m the protagonist of some light novel. Hahaha!

In fact, that made me even more excited. Now that I thought back, I had probably lost my cool then. That’s why, when the lady who was to be my stepmother came to visit with her daughter the next day, it felt like I just got a bucket of ice-cold water dumped over my head.



The one standing over there was Ayai Yume.


By that time, she had turned into Irido Yume.

The two of us staring at each other with our mouths wide open were surely shouting this same words on the inside.

—Damn you, God!!

And so, my ex-girlfriend became my stepsister.


“……Thanks for the meal.”

Ayai—or rather, Yume spoke curtly and stacked the tablewares. She then proceeded to the kitchen.

……Damn it. What bad timing. I had also just finished my meal. It would be weird of me to just sit down here.

“Thanks for the meal.”

I also stacked my tablewares and brought them towards the kitchen—where Yume was washing her dishes.

She shot a glance at me but did not said a thing. She just continued cleaning wordlessly.

I stood next to her and washed my dishes silently as well.

If possible, I would not have wanted to line up next to her like that as well, but it would be a problem to avoid each other suspiciously. After all—

“Looks like I’ve been worried over nothing for a boy and girl of this age to become siblings all of a sudden. Fortunately, it seems like they have getting along just fine.”

“Indeed! I even saw Mizuto and Yume going to the bookstore together today too. I guess having a common interest sure helps them to bond!”

“That had been my greatest worry these past few days, but it seems I can breathe easy for now.”

Dad and Yume’s mother were having a joyous conversation in the living room.

The currently remarried couple seemed to living in happiness everyday—completely opposite to us children.

“……You understand, right?”

“……About what.”

Yume spoke in a low voice that seemed to be drowned by the flowing water next to me.

“Don’t ever make the two of them feel any regrets.”

“I know that. I was always planning to take the matter about us to the grave.”

“That’s fine then.”

“……You’re so obnoxious in every way. Since when did you become like this.”

“If I was any different from the past, then it’s 100 percent your fault.”


“What’s wrong about that.”

“Hey! What are you guys talking about?”

Dad called out from the living room, and we instantly masked our feelings.

“Oh, nothing much, we’re just talking about the novel we bought today, you know.”

“Right, we were just discussing about that novel.”


Yume replied to Dad in a bright voice, while giving me a low-kick where she could not be seen.

“(There’s no need to add that, ‘you know’. Is your language even okay?)”

“(No need for you to worry about that. You should know that my language in the top 100 placing nationwide.)”

“(……Grr. I can’t believe myself for even saying that was amazing.)”

“(I should feel indignant. To think I was actually happy to hear that then.)”

On the surface, we played the roles of siblings with a good relationship.

There was no way we could let Dad or Ms. Yuni know about our previous relationship and cause them regrets—

That was the only matter for which me and Yume could see face-to-face.

But if you turned that on its head, it meant that there was not a single other thing on which we could agree on.

After returning to my room, I started to read the novel I bought today when I heard the sound of knocking.

“Dad? What is it?”

There was no reply.

I was annoyed to be disturbed while I was reading, but I also did not want to ruin the good mood they had from the remarriage—so I slotted a bookmark to the page I was reading and opened the door.

The one standing there in the corridor was the girl I loathed the most in this world.

In other words, it was Yume.

“……What is it.”

I greeted her with a tone 100 degrees colder than before.

She only snorted with a hmph, as if she was implying that that was nothing.

To speak in no unclear terms, I wanted to send this girl flying.

“There’s something I want to talk about. You free now?”

“No I’m not. You should know what I bought just now.”

“I know. That’s why I’m here, I already finished reading it.”


Looks like she purposely came to disturb me while I was reading.

Ever since when we were dating, this girl had always read slightly faster than me. Whenever we bought the same book and started reading at the same time, she would always end up finish reading the book just as I was about reaching the climax of the story.

How underhanded.

That’s what I hate about her.

Thank goodness we broke up.

“……What do you want. Spit it out.”

“Let me in. I don’t want to let them hear.”


“How about you don’t click your tongue over every single thing?”

“I’ll do so if you’d just disappear from my sight.”


I carefully checked the corridor to make sure that neither Dad nor Ms. Yuni were around before letting her in.

Yume looked downwards and entered my room.

“What a filthy book-filled room. It feels like I’m becoming dirty just by being here.”

“That isn’t what you said the other time you came over when Dad went on a business trip though. ‘Wow……! What a collection!’, was it?”

“That is a thing of the past. Currently, all I feel from seeing that entire collection of Sherlock Holmes on that shelf is nothing but irritation.”

“Feel whatever the heck you want.”

I heaved a sigh and sat down on the bed half-covered in books.

“So, what do want to talk about?”

“I’m nearing my limit already.”

Yume spoke coldly while still standing.

“I can’t stand it anymore—Just how much longer do I have to bear having you call me so familiarly by my first name?”

I furrowed my eyebrows.

There was no need for me to hide my displeasure at her.

“I can say the same for you. Aren’t you also calling me by my first name.”

“That I can still bear with. But, I just can’t stand having someone like you call me by my name. I haven’t even allowed that when we were dating—during middle school.”

So, she doesn’t even want to speak of ‘when we were dating’, I see I see.

“There’s no other choice since we have the same surname now, isn’t it. How else should I call?”

“There’s another option, isn’t it?”

“Which is?”

“‘Big sister.’”


“Since we’re siblings now, there’s nothing wrong with you calling me ‘big sister’, isn’t it.”

“No no no, hold on a minute.”

I had to hold my head with my hands.

“Call you? Big sister? ……Stop being ridiculous, shouldn’t it be the other way around instead.”


“It’s ‘big brother’. That’s how you should call me. It’s obvious that I’m the elder one.”

What kind of nonsense is she spouting now.


Yume shrugged her shoulders and sighed before walking towards a bookshelf filled with colorful covers.

It was the shelf containing light novels.

She picked one out from the shelf, which had the illustration of a beautiful girl on top, and made a look of pity.

“I should’ve expected that reading such things would’ve made you hold some irregular expectations with regards to ‘stepsisters’. How lecherous. Just what are you expecting of me?”

“You dumb broad. Then you might as well say people who read mystery novels will become killers.”

I don’t mind if she insults me, but I won’t stand for her insulting the books I own.

She returned the light novel (which did not depict any sister, much less stepsisters) to the shelf and turned towards me.

“Enough with the jokes, it’s impossible for me to the little sister. Not way in hell. Just think about it—I’m your elder sister.”

“What kind of universe’s logic are you using. Perhaps you need to worry about your own language skills.”

“This is not about language, but a simple problem about counting. Need I even mention that I’m in the top 100 placing for mathematic nationwide.”

I knew that already. This girl excelled in math more than the languages, a behavior unbecoming of any bookworm. Unforgivable.

“The one who is born earlier would be the elder sibling. That is the first conjecture. And I was born earlier than you. That is the second conjecture. Thus, I’m the elder sister. End of proof. Understood?”

Her area of specialization was more logic rather than arithmetic, but there was something I couldn’t let pass.

“……If my memory serves me right, our birth date seems to be on the same day, however.”

That’s right, this was another one of God’s trap.

This girl’s birthday was on the exact same date as mine.

Which wasn’t to say that we were fated for each other, but we did agree on the repulsive notion of celebrating our birthdays together, and I can’t say there wasn’t memories of us exchanging presents as if in a devilish ceremony. Anyway, those memories were now in the trash bin.

“That’s why there’s no such thing as an elder sibling between us.”

“I remember clearly declaring to you that ’m the elder sister though.”

If you were to ask me, I’d prefer a younger stepsister over an elder stepsister. Not that I have any ulterior motive, that is.

“In any case, there’s no denying the facts. It is only the date which matches—you can’t say the same about the time of birth.”

“Time of birth?”

“I’ve already looked into it.”

Speaking like a detective towards a suspect, she brought out her smartphone and shoved the screen before me.

“Look at this.”

The screen showed the picture of a baby. And under the picture were several words.

“Your time of birth was 11.34am.”

She slid her finger across the screen to bring up the next image. Similarly showing the picture of a baby, she pointed to the clock photographed in it.

“And as you can see from this picture, I was born at least on or before 11.04am. Thus, I’m the elder one by at least 30 minutes. Now you get it?”


Is this girl serious.

For the sake of something like this, she had gone through the picture albums in my house just to find the time of birth.

“Way too creepy.”

When I honestly stated my opinion, she instantly turned red.

“Wh-…… Why!? The perfect evidence is needed for a flawless deduction, isn’t it!?”

“There it is, that obsession for classical mysteries. If you like puzzles so much, just go do some puzzles. Don’t go expecting that from novels.”

“Ah! You said it! You totally said it! You just totally dissed classical mysteries, didn’t you!!”

It’s not like I disliked classical mysteries. I just hated this self-proclaimed mystery geek who can’t stop going on about a sense of puzzlement. More specifically, I just hate this girl.

Yume’s cool attitude crumbled and her flushed red as she trembled.

“…………Even though you were praising me about how smart I was back then…………!!”

……Is she going to go there?

Didn’t the two of us treat those times as never having happened?

I scratched my cheek awkwardly. ……I understood the feelings of anger at having the things you like being mocked more than yourself, having just felt that.

I heaved a deep sigh as I stood up—

And lightly tapped Yume’s head.

“Fine fine. You’re amazing. You’re so smart, pretty much a genius. You can be the elder sister.”

Somehow, it felt nostalgic. In the past, whenever something happened, I would always see Ayai making this bashful expression—

However, the current Yume was not being bashful or anything.

Her whole body was shivering, much like a volcano on the verge of eruption—



“That part of you that immediately gets what people want and just do so without any reservations is exactly what I hate about you!! You idioooooooot!!!”

She shouted as she ran out of the room knocking over a few towers of books.

I stood at the same spot in a daze. ……I have never seen that side of her before while we were dating.

“Mizuto? Was that Yume I hear shouting?”

Ms. Yuni’s voice came over the corridor and I frantically made up some excuse.


She just selfishly comes over, selfishly raise a racket and selfishly runs away, leaving me behind to clear up the mess—and in the end, even makes a face I don’t know.

As I said before, that’s the part I hate about her.


In the end.

“……Good morning, Mizuto.”

“……Good morning, Yume.”

The way we called each other did not change.

As to be expected, I was still great reluctant to call her by ‘big sister’.

And even though I didn’t address her that way, she also did not point me out. Perhaps she may also feel awkward at having me call her that way for all I know.

……In any case.

“Mizuto, can you pass the soy sauce?”

“Sure, here you go, Yume.”

For the two of us who had stuck to using our last names when we were dating, we were now calling each other by our first names—how ironic was that indeed.

This must have been God’s trap as well.

If that was so, then I definitely owed God or whoever it is my clenched fist.

And at that time—yeah.

I’ll probably invite along this disagreeable mystery-loving geek.

After all, I’m the kind of person who can immediately tell what others want and take action without reservations.

Table of Contents | Forward to Chapter 2

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