Yukino Natsuki

The protagonist's sister (cousin).A first-year student at Kotogaoka Academy.

From Koichi's perspective, she is his cousin.
Having lost both her parents, she was adopted into the Yukino family.

Originally, only Koichi was supposed to move to the island, but she later followed him here.
It seems she acted out of the belief that Koichi couldn’t manage without her.
That said, Koichi is generally the more dependable one, so her reasoning doesn’t hold up well in practice.

She is fairly proactive, though not particularly tomboyish in nature.
She tends to be the caring type and is somewhat quiet—especially around others—but shows a more lively side only around Koichi.

She takes interest in many things and has numerous hobbies.
However, her tendency to lose interest quickly is her Achilles' heel.