Kosei and Neito appear to have a friendly relationship. During the U.A. Sports Festival Arc Kosei went along with Neito's plan to throw the qualifier. He also joined Neito's team for the Cavalry Battle, during which, he joined his classmate in taunting Katsuki Bakugo when briefly subduing him with his Quirk. 
Kosei and Sen appear to be good friends, as mentioned in Volume 22. The two have teamed up during the U.A. Sports Festival and followed Neito Monoma's plan on overthrowing Class 1-A. Kosei is also one of the few who can make Sen smile at times when he's not in his stoic persona. 
When Kosei got restrained by Tsuyu and dragged off to Team 1-A's prison, it appears that he presumably developed a small crush on her. While getting captured, he seemed to be mostly thinking about how Tsuyu's tongue was wrapped around him.
- ↑ My Hero Academia Anime: Episode 18.
- ↑ My Hero Academia Anime: Episode 17.
- ↑ My Hero Academia Manga: Chapter 196.