Kei Yūki first appears when his motorcycle nearly strikes C-ko on the street, prompting A-ko to knock him down. He later meets A-ko at her fast-food restaurant job and offers her a ride to reunite with C-ko, who harbors animosity toward him. Kei carries an earring C-ko previously dropped.

His background involves an arranged marriage orchestrated by Hikaru Daitokuji, betrothing him to their teacher Miss Ayumi. This engagement ignites fierce opposition from both A-ko and B-ko due to their rivalrous affections for him. The wedding ceremony is planned for Graviton City High School for Girls' gymnasium.

During the event, an alien fleet led by the Alpha Cygnan Queen arrives to halt the marriage. Kei, Ayumi, and C-ko become entangled in the chaos as A-ko and B-ko disrupt the ceremony and battle each other. Ayumi’s pendant—identical to an artifact from ancient Babylonian ruins—ties into an alien prophecy foretelling the return of their lost princess.

After the alien threat resolves and C-ko chooses to remain on Earth, the conclusion of Kei’s engagement to Ayumi remains undetailed. His role centers on being a focal point of romantic rivalry and his participation in the arranged marriage storyline.