In this world, half-siblings are surprisingly common. With a male-to-female ratio of 1 in 100, it's not unusual for a single man to have a hundred children. So naturally, such situations arise often.
Paternity isn't typically verified retroactively with genetic testing. Instead, it's considered important for everyone to know from the start. The sperm provided through government-supported pregnancy and childbirth programs is used randomly and anonymously, under the pretense of avoiding inbreeding and preserving genetic diversity. There's no label indicating the donor. In fact, there are even rumors that foreign donors are included, which explains why some children occasionally have features that don't look particularly Japanese.
So in this world, being recognized as a half-sibling only happens when the biological father personally acknowledges the child as his own.
That makes these kinds of familial relationships somewhat rare—mostly occurring in harem-like, insular social settings—and often leads to siblings who are unusually close.
Our biological father is a man named Kohaku Umino. A legendary sperm donor, he is said to have personally fathered over 2,000 children.
He also had a hobby: birdwatching. So naturally, all of us were given bird-themed names. It's the kind of thing that gives away our origins immediately—simple, and honestly, a bit obvious.
Unfortunately, he's already passed away, so his progeny won't be increasing any further.
Beep.
While out on official rounds, I spotted a very familiar face and instinctively gave a short honk before calling out.
"Hey, what are you doing, Misago?"
I got out of the car and approached her with the same casual, flirty tone I always use. It's our usual vibe.
"I'm giving directions. If you're going to get in the way, please leave..."
Her voice was laced with that unmistakable "you're being a huge nuisance" energy.
"Oho, do I sense murderous intent?"
I played dumb, like always. Apparently I can be intense, but honestly, coming from a younger sister, it's kind of adorable. We're in the same school year, so it doesn't feel like there's much of a difference, but even six months can change someone's outlook.
"Sorry, but I'm kind of on a date right now. Maybe later?"
A voice I didn't recognize suddenly spoke—low and a little mature. I blinked in surprise.
"Eh…?"
Our voices overlapped perfectly—Misago's and mine. That's sibling instinct. In fact, we're often told our dynamic is too similar and a bit much to handle.
Looking for the voice's owner, I noticed someone standing just behind Misago, partially hidden from view.
A small figure, dressed in a white yukata… A boy? Wait—a boy?!
I had to double-take.
"Did I ruin the moment?"
He had a playful tone, like he was enjoying the situation. His speech had a hint of maturity that didn't quite match his childlike appearance.
In response, Misago blushed bright red. Cute—my sister really is a pure one.
"Where'd you find this guy, Misago?!"
I couldn't help shouting it out, right there on the coastal road. Not out of anger—Misago didn't even flinch, after all.
So what exactly happened? I was out handling some minor errands, spotted Misago while driving, and figured I'd say hi. That's when I saw her—hand in hand—with a boy.
And then the boy said they were on a date.
My same-age, half-sister. On a date.
Of course I'd yell.
"I picked him up over there."
Misago motioned with a "shh" gesture, pointing toward the rocky beach below the cliff.
I followed her gaze. The waves crashed violently, tumbling rocks and making a sound like rattling bells. This coastline is known for being treacherous—there are actual records of it claiming lives.
"Let's just call it an emergency rescue, okay?"
Even Misago looked puzzled. Clearly, this situation was beyond explanation.
"Not my department, though."
My job today was just routine municipal grunt work. The population around here has been seriously declining, so I was out checking on vacant houses and other odds and ends.
As for Misago finding a random boy from who-knows-where—that definitely sounded like a job for the police, not me.
"If anyone asks, I'll say it doesn't seem like a criminal matter."
At that, Misago's face brightened.
"Thanks."
"Be sure to share the leftovers, okay?"
I replied lightly, fluttering my hand and pointing vaguely at my stomach—like, "right here."
"Well, if it all works out, maybe."
She murmured with a small smile.
And that was how we met Hisui that day—on the very day he first appeared in our town.
Author's Note :
This chapter is actually told from Tsubame's point of view. I realized I couldn't wrap up Day 1 cleanly without it, so I'm slipping it in here at the last minute. Everything should come together in the next episode.
This novel has been lovingly translated by https://www.patreon.com/Jenuinez_translation