Prologue

The late summer evening draped itself over the room, warm and heavy, as I lay sprawled face-down on my bed, a manga open beneath me. On my back, the familiar weight of another person pressed down, accompanied by the incessant chirping of a mobile game.

“Akira, come on, get off already. You’re getting heavy, and I’m exhausted…”

“Just a bit longer like this—hey, Aniki, don’t shake so suddenly! I messed up my game!”

“Ha, my bad…”

The sounds of Akira’s mobile game had been relentlessly blaring above my head for a while now. I, Majima Ryouta, a second-year high school student, was pinned beneath my first-year stepsibling, Akira, who lounged face-down atop me, engrossed in their game. Why was I sandwiched between Akira and the bed like this? According to them, “Aniki’s back is my best spot, you know.”

To begin with, this level of closeness felt… a bit unnatural for high schoolers. Awkward, even. I tried to play it cool, but awkward things are just awkward. “How much longer?” I grumbled.

“Maybe five more minutes?” Akira replied breezily. We’d been having this same exchange for over twenty minutes now.

“Ugh, lost again…”

“Then get off already.”

“No way. One more round.”

“Oi, Akira, I’m at my limit here—”

“Whoa, Aniki! Ugh!”

I propped myself up on my arms, and Akira, caught off balance, tumbled off me, their light body bouncing slightly on the bed.

“Ah, my bad!”

“Jeez! Don’t stand up so suddenly!”

“Sorry, I didn’t mean to…”

“If you’re really sorry… how about we sleep together tonight?”

“How does that even follow?!”

“It’s fine, right? Just sleeping together? I’ve always wanted to sleep like ‘siblings’ do.”

“No, no, it’s not fine… At our age, sleeping together as ‘siblings’ is…”

Akira’s mischievous grin gleamed as they looked at me. “…Aniki, you’ve done all sorts of things to me, haven’t you? The least you could do is grant me this one request.”

“No, about that, I’m really sorry… But, uh, sleeping together is a bit…”

“…Just kidding, that was a cheap shot. Sorry, Aniki, I put you on the spot, huh?”

Relief washed over me as I muttered, “So it was a joke,” but then Akira slipped under my blanket with a sly grin.

“Oh well, good night then~”

“Oi, Akira.”

“Munya munya~”

“Don’t sleep in my blanket…”

I can’t help but think, if only they were my little brother. If they were my brother, it’d be fine. If they were my brother, letting them play their game on my back would be no big deal. If they were my brother, even if I shook them off my back, I probably wouldn’t need to worry about it. If they were my brother, sleeping together… might be okay.

But as I watched Akira pretend to sleep, I tried swapping “if they were my brother” with “because they’re my sister.” If the outcome stayed the same, no problem. But if it changed entirely, it meant I no longer saw Akira as just a brother—or even as the same gender.

Just a few days ago, I’d been interacting with Akira under a misunderstanding I couldn’t just laugh off. How had I even gotten it so wrong? And if I told someone about it, how would they react?

—If I said that Akira, who I thought was my stepbrother, turned out to be my stepsister…

They’d probably laugh or stare in disbelief. That much was obvious. But what I really wanted to know was what came after. How would people see me and Akira? No—forget others for now. The real issue was my relationship with Akira.

How was I supposed to act around this carefree, brotherly-close stepsister? As her older brother, I’d been agonizing over this for days.

Anyway, Akira.

You’re not my stepbrother.

You’re my stepsister.

A girl.

Way too cute.

And, honestly, such a handful…