Kid was the friend of Ryou Kuroyanagi. They met while he was attending high school in America. Ryo reflects on this in episode 26 of the anime. Kid was an older classmate and Ryo's best friend.
Others used to make fun of Ryo for being an overachiever, because he skipped so many grades (he ended up graduating from a Harvard program at 19). Kid told him not to worry about guys like that, that they were jealous that they couldn't compare to Ryo's genius.
Kid stood up for Ryo and helped him fight off bullies. He also taught him how to flirt with women.
He would also listen to Ryo's complaints. They were together in whatever they did. He did anything for Ryo, without ever making it seem like he didn't want to.
Ryo also makes some claims about Kid which may not be true, since during his recollections about Kid, Kid himself says "no, I didn't", indicating that Ryo was embellishing. These include:
- performing a medical examination on Ryo when he was weakened in body and mind after too much studying
- taking the place of Ryo's mother when he was homesick
- raise money (probably stolen) and give it to Ryo when he was broke
In the end of Ryo's story, it turns out Kid failed again (he wasn't that bright). In the end, Ryo clasps Kid's shoulders and dubs him a Double Classmate. This is shortened to "Double Crust", the name of Kyo's dish that inspired his recollection.