Sui Takishima, Kei Takishima's younger brother, bears a striking childhood resemblance to him. Initially consumed by resentment and jealousy, Sui views Kei as a "robot wearing human skin" due to his flawless academic prowess. Constant comparisons fuel Sui's deep inferiority complex, convincing him he can never match his brother. He rebels by neglecting studies, rejecting tutors, and deliberately underperforming academically as defiance.

Enrolled in Hakusen Academy's elementary division as a sixth-grader in class B, Sui maintains average grades—a reflection of his apathy rather than ability. His non-cooperation drives away numerous tutors, causing significant family concern, particularly for his father.

Sui's perspective shifts after encountering Hikari Hanazono, whom he nicknames "Stupid Girl." Through her, he witnesses Kei's vulnerabilities—desperation and genuine care—contradicting his emotionless perception. These humanizing traits help Sui reconcile his inadequacy. Resentment transforms into fervent admiration, evolving into a brother complex marked by overt affection and pride.

This loyalty solidifies: Sui proactively enlists Hikari's aid for Kei's benefit and vocally champions his brother. Though his mischief endures, it now pairs with protectiveness and support. Sui ultimately overcomes his inferiority complex, fully embracing his bond with Kei.