KoSen

KoSen is the slash ship between Kosei Tsuburaba and Sen Kaibara from the My Hero Academia fandom.

Canon
In the anime, the only time these two are seen together is when they teamed up at the sports festival, both agreeing to follow their classmate Neito Monoma’s plan to win (they lost). In the volume 22 of the manga, it has been stated that the two are close friends, Kosei being the only one who can occasionally make Sen smile when he is out of his stoic personality.

Fanon
KoSen is the most popular ship for Sen, and Second for Kosei, after Tsuyu Asui.

Sen is introverted and a bit apathetic, Kosei is one of the few who understands him and doesn't judge him for his quirks. Leading to a close relationship and eventual romantic one.

On AO3, KoSen is the most written ship for both Kousei Tsuburaba and Sen Kaibara.

Fandom
FAN FICTION 

TUMBLR 

Trivia

 * Kosei is one of the only people Sen respects and admires.