Kei Yuki, a human character in *Project A-Ko 4: Final*, first crosses paths with C-ko during a near-miss motorcycle incident, where A-ko intervenes and knocks him down. He later returns an earring C-ko dropped, despite her dislike for him. His central role emerges when Hikari Daitokuji orchestrates his arranged marriage to their Graviton City High School teacher, Ayumi. This engagement becomes a pivotal conflict, as both A-ko harbors feelings for Kei and B-ko's obsession with C-ko fuels her rivalry with A-ko over his attention. Their wedding transforms the school gymnasium into a chapel.

During the ceremony, Kei, Ayumi, and C-ko become engulfed in escalating chaos: the destructive melee between A-ko and B-ko, an invading alien fleet, and the Earth Defense Force's intervention. Kei's presence intensifies the clash between A-ko and B-ko, though he remains a passive figure amidst the larger battles. The narrative provides no details on Kei's reactions to the alien incursion or the final outcome of his engagement. Information regarding Kei's broader background or development in other media is not covered.