Keita Suminoe is a 15-year-old first-year student at Gakushū Senior High, previously introduced preparing for entrance exams as a third-year junior high student. He belongs to a blended family; his father remarried after Keita's biological mother died in his childhood, giving him two older twin stepsisters, Ako and Riko Suminoe. These sisters, one year his senior and sharing no blood relation, were raised alongside him from a young age. Keita sports spiky blue hair, blue eyes, and an athletic build from past track and field involvement. Academically, he ranks below Ako but above Riko. He initially planned to use his athleticism for a scholarship but chose to attend the same high school as his sisters to prevent their loneliness.

Keita displays a conflicted personality, often acting as the most mature sibling despite being younger. His strong morals gradually erode under the persistent romantic and sexual advances from Ako and Riko. While he initially resists their attention, he eventually becomes proactive in his own desires, indulging when pushed beyond his limits. Childhood infatuation led him to spend all his savings on a prayer to marry both sisters, abandoned only after learning polygamy is illegal from his father. He remains fiercely protective of them, opposing others who speculate about their relationship and insisting no one else sees them in lingerie. Internally, he wrestles with guilt over his attraction, yet acknowledges loving them as both siblings and romantic interests.

His relationships extend beyond family. He befriends Miharu Mikuni, a shy classmate who returns his entrance exam results. Their bond strengthens as Keita defends her from misunderstandings, developing mutual attraction; Miharu later declares herself his girlfriend after he breaks up with Yūzuki Kiryū, his homeroom teacher. Keita's relationship with Yūzuki evolves from mutual hostility into a secret romantic involvement involving dating and plans for intimacy, but ends definitively over Keita's lack of future planning. He also interacts with Mikazuki Kiryū, Yūzuki's younger sister who harbors a crush on him, though he treats her platonically. Friends include Toda Edogawa, a male classmate who occasionally stays at his home, though Keita resents Toda's assumptions about the household.

A key trait is his vulnerability to alcohol: drinking makes him lively but erases memories upon sobriety, though recollections return if he drinks again. His character develops significantly. After entering high school, he navigates romantic entanglements with his sisters, Miharu, and Yūzuki, culminating in the breakup with Yūzuki and a committed relationship with Miharu. Protective instincts drive him to confront peers or authority figures concerning his sisters, and he assists Riko in overcoming exhibitionist anxieties for her gymnastics. By the manga's later chapters, he reconciles his feelings, openly pursuing his attractions while maintaining his role as a mediator in familial conflicts.