Eguchi Yuu

The main protagonist. The youngest child of the Eguchi family. Shortly after birth, he was abandoned at Harmonica Nursery School and taken in by the Eguchi family, who ran the nursery.

He became the eldest son of the Eguchi household, with three older sisters. Because of this, his feelings and affection for his family are stronger than most.

Since childhood, he has admired each of his three older sisters. He now seems to harbor a faint affection for the third sister, Miki, though he remains oblivious to it himself.

At school, he is forcibly made to handle the Student Council's odd jobs by Miki, the Student Council President.

"My... thing has the Golden ratio? We've known each other since birth, ...but I didn't know that."