Tetsuhiko Kai is a high school student at Hodsumino High School and a close friend of Sueharu Maru. He possesses light brown hair and a mischievous, boyish charm, though his behavioral tendencies earn him a poor reputation among female classmates.

Kai combines intelligence with manipulativeness, finding amusement in observing and orchestrating chaotic situations, particularly Sueharu's romantic entanglements. He schemes for personal amusement or benefit, like recording and profiting from Sueharu's viral public rejection video. He holds a notorious reputation as a "scumbag" in romance, having three-timed classmates before expanding to seven-timing girls from other schools after local ostracization. Despite his self-serving actions, he demonstrates loyalty to friends like Sueharu and Rena Asagi, occasionally intervening to protect them or offer strategic advice, such as urging Sueharu to prioritize high school over an acting comeback.

Kai conceals significant family complexities: Shun Hardy is his biological father, and Rena Asagi is his half-sister, relationships kept secret due to unresolved tensions. He founded the Ultramarine Alliance (Gunjo Alliance), handling video production and editing to manipulate narratives around Sueharu's life. His name etymologically references "Cain" from the *Fire Emblem* series, reflecting themes of strategy and conflict.

Across the series, Kai evolves from a troublemaker into a behind-the-scenes mediator, proposing compromises during conflicts involving Sueharu's career decisions and romantic disputes. While his manipulative tactics persist, his underlying concern for his friends' well-being becomes more apparent, particularly in interactions with Rena and efforts to navigate Sueharu's challenges.