Jun'ichi Nagase, a second-year high school student, carries the notorious middle school nickname "Geno Killer" from his delinquent past. This reputation traces back to a troubled childhood after his parents and sister departed to travel the world following elementary school, leaving him to live alone for reasons he never fully grasped. His conduct shifted when his sister Minato returned to live with him. Physically, he stands 184 cm tall with dark brown short hair parted to the side and teal, slender-shaped eyes. He maintains a slim, pale build and favors school uniforms or casual wear like jeans and sweaters, always wearing a pendant necklace. His possessions include a bicycle, gaming console, and mobile phone. His personality merges sarcasm, laziness, and protectiveness. He routinely skips homework for gaming and relies on Minato to avoid oversleeping. Despite a blunt, streetwise exterior, he displays kindness and optimism. Peers and teachers often tease him, partly over perceived romantic tension with Minato—around whom he exhibits flustered reactions while denying any feelings. He engages in self-deprecating internal monologues and breaks the fourth wall. His life twists when he rescues Yuuhi Katagiri from harassers. A misunderstanding leads him to kiss her publicly at school, sparking her hostility. Their families soon reveal an arranged marriage, forcing them to cohabitate with Minato. Despite early friction, Jun'ichi repeatedly protects Yuuhi from threats, and their relationship evolves into mutual, unspoken romantic feelings. His narrative diverges across adaptations: The anime sees him gradually acknowledge deeper feelings for Minato, confessing he cannot live without her. The manga concludes with him reciprocating Yuuhi's affection, entering a romantic relationship. The original PC visual novel confirms Minato as his blood-related sister, while PS2/PSP adaptations and the anime imply she may be adopted. The OVA includes non-canon comedic segments, such as mecha battle dream sequences. Beyond romance, he shares a close yet antagonistic friendship with Fuyuhiko Nishino, whose schemes frequently entangle him in trouble. He interacts regularly with classmates like news club member Tsukasa Kiryu and student council president Mitsuki Shiina, who exploits him for tasks. Parental abandonment lingers as a theme; his absent parents remain financially supportive, occasionally sending gifts like birthday presents. Their action-oriented lifestyles starkly contrast his domestic struggles.