"Miki's coworker at Bobson's ice cream parlor who is a talented but troubled pianist. He also falls in love with Miki, and his methods to get her away from Yuu almost bring him success after he uses Miki's lost medallion (which had a pictue of Yuu) to fabricate a lie about Miki's true feelings for him. However, this also fails after Yuu makes a trip outside of Tokyo but cannot enjoy since he misses Miki too much, and Miki forces herself to see the worst side of herself. When they see each other again, they come back together.

A much more mature Kai reappears later, offering Miki his sincere and unconditional support after Yuu goes to study abroad. He says he does not need to have Miki forgetting Yuu completely, since he's happy to help her through her ordeal. Miki appreciates his company and even dates him briefly, but he cannot compete with Yuu. It is hinted at the end that he may or may not get together with Suzu Sakuma."

(From Wikipedia)