Kai Renge belongs to the immensely wealthy Renge Family and attends the exclusive Kuge course for elite students at Hojo High School. Driven partly by envy seeing peers secure lifelong partners early, he selects financially struggling ordinary course student Yuzuru Hagino as his "Honey," requiring her service in exchange for tuition exemption.

Initially, he treats Yuzuru as a disposable toy, subjecting her to frequent sexual harassment and verbal degradation. This includes forced kisses, public groping, and invasive demands, framed as revenge for a past incident during her summer job. He openly refers to her as a purchased possession, emphasizing their relationship's transactional nature. Kai demonstrates notable athletic skill, excelling in basketball and tennis, often showcased in school competitions like the "Drops Tournament," where he pressures the unskilled Yuzuru to perform.

His character evolves as genuine feelings develop. He becomes protective, shielding her from physical harm during a basketball game and injuring his hand to prevent her collision. This shift includes possessiveness; he declares he will never release her from the Honey role. He confronts rivals like Chihaya Yurioka, who schemes to separate them, resorting to forceful measures like invoking an archaic "Ribbon rule" to reclaim Yuzuru after Chihaya manipulates her contract transfer. His protectiveness extends to defending her against Chihaya's psychological torment and attempts to undermine their bond.

His background reveals complexity; despite initial cruelty, his actions are rooted in a deep-seated need for genuine connection, contrasting with his peers' earlier successes. His proposal during an intimate bath scene marks pivotal commitment, culminating in their marriage. Throughout, he balances a stern exterior with vulnerability, expressing fear of losing Yuzuru and insisting she remain his Honey despite external pressures.