KoSen

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

Canon
Kosei and Sen are seen together a number of times in the anime and manga. They both appear a lot together in the Sports Festival. In the Sports Festival, they teamed up together and Sen is seen smiling at Kosei. A few moments later, they both teamed up with Neito Monoma and Shihai Kuroiro. The boys agreed to follow Neito's plan to win. Neito and Kosei taunt Katsuki Bakugo, leading to Katsuki getting enraged and stealing their team's points. In the end, team Katsuki steals all team Neito's points and Neito's team loses.

Kosei and Sen appear quite a few times in the manga as well as the anime. When class 1-A and class 1-B was hanging out together, They both are seen sitting together in the background. Sen and Kosei are also both in the same internship, with Togaru Kamakiri. In volume 22 of the manga, it is stated that Kosei is one of the only people who can occasionally make Sen smile when he is out of his stoic persona.

Because the boys are background characters, they don't have a lot of screen time. Due to their lack of screen time, they aren't seen interacting a lot together. However, when they have screen time, they are often seen together. They are both sometimes seen together in the background so a lot of people don't pay a lot of attention to them. The only times they weren't seen together in the background was in the Sports Festival.

Fanon
KoSen is the most popular ship for Sen, and Second for Kosei, after Tsuyu Asui. The ship started to gain more popularity when it was revealed that Kosei and Sen are best friends. Shippers think it's cute. Since they both don't have a lot of screen time, the ship isn't that popular. Sen is introverted and a bit apathetic but Kosei is one of the few who understands him and doesn't judge him for his quirk, leading to a close friendship and eventually a romantic one. On AO3, KoSen is the most written ship for both Kosei Tsuburaba and Sen Kaibara.

Fandom
FAN FICTION 

TUMBLR 

Trivia

 * Kosei is one of the only people Sen respects and admires.
 * In the Ultra Analysis Book, on Sen Kaibara's page, in relationships, Kosei says he's very comfortable around Sen, and Sen says Kosei is easy to talk to, hinting that they are close.