Kei Yuki, a motorcyclist, first crosses paths with the main characters during a spring break incident when C-ko darts into the street, nearly colliding with his bike. This near-miss causes A-ko, rushing to save C-ko, to knock Kei off his motorcycle, sparking a brief scuffle. Amid the chaos, a glass slipper earring is dislodged, later becoming an identifying token.

Notably reserved, Kei communicates through actions more than words. His appearance as a handsome motorcyclist reignites tension when he visits the fast-food restaurant where A-ko works part-time. Both A-ko and B-ko develop fierce, competitive attractions—A-ko seeing him as a potential boyfriend, B-ko aiming to steal him from her—culminating in a disastrous group date at the Alpha Cygnans' Space Hotel.

Despite their rivalry, Kei's affections fix solely on C-ko. He confesses during the space hotel turmoil, but C-ko rejects him with feral hostility, snarling and hissing due to jealousy over his closeness to A-ko, devastating both rivals. His role expands in *Project A-ko 4: FINAL*, where Miss Ayumi enters an arranged marriage with him, orchestrated by B-ko's father, Hikaru Daitokuji. A-ko and B-ko conspire to sabotage the wedding to reclaim him, though Kei maintains his passive, silent demeanor.

Throughout, he functions as an object of romantic pursuit and a catalyst for conflict among the central trio, with no elaboration on his background or motivations beyond these events.