Eiji Kai is a character who appears exclusively in the anime adaptation Kimikiss Pure Rouge, having no counterpart in the original video game upon which the series is based. He is a third-year high school student and a classmate of Mao Mizusawa. Eiji is distinguished by his brown, ear-length hair and purple eyes. His primary talents are musical, as he is a saxophone player who also works at a jazz bar, indicating a deep connection to music and a more mature, nightlife-oriented environment outside of school.

Initially, Eiji is presented as a loner who does not get along with Mao Mizusawa. This conflict creates an early friction between them. However, as they interact, Eiji gradually opens up, and his personality reveals a more approachable side beneath his solitary exterior. This change allows for a romantic relationship to develop, and he and Mao begin dating. His role in the story is central to the romantic development of another main character, Kouichi Sanada. Eiji becomes the catalyst for the central conflict in a significant love triangle, as he is the one who dates Mao, forcing Kouichi to confront his own long-held feelings for her.

Eiji's key relationship is undeniably with Mao Mizusawa. Their connection evolves from animosity to romance, and they become a couple for a period. This relationship is meaningful for Eiji, as it shows his character growth and his capacity for emotional intimacy. However, this relationship is also the source of his primary role in the plot. Eventually, Mao breaks up with Eiji after realizing and admitting to him that her true feelings belong to Kouichi Sanada, her childhood friend. Following the breakup, Eiji demonstrates significant maturity. He remains friends with Mao and is later shown being willing to have a sincere conversation with Kouichi about her. In this conversation, Kouichi asks Eiji to take good care of Mao, showing a mutual trust and respect that has formed between the two young men. This interaction cements Eiji's development from a reticent outsider to an understanding and pivotal figure in the resolution of the main romance.

Beyond his personality and relationships, Eiji has no supernatural or extraordinary abilities. His notable skill is his talent for playing the saxophone, a trait that defines his character design and his after-school life working at a jazz bar. This musical ability reinforces his artistic and somewhat reserved nature.