Kosei Tsuburaba/Relationships

Neito Monoma
Kosei and Neito appear to have a friendly relationship. During the U.A. Sports Festival 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.

Sen Kaibara
Kosei and Sen are good friends. The two have teamed up during the U.A. Sports Festival and followed Neito'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.

Shoto Todoroki
During the Vanguard Action Squad Invasion, Shoto carried Kosei to safety when the latter had been rendered unconscious thanks to Mustard's Gas. Kosei has respected Shoto ever since then, suggesting that he's grateful to the dual Quirk user for having saved him.

Tsuyu Asui
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.