KoSen

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

Canon
Sen Kaibara and Kosei Tsuburaba 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 agreed to team up with each other. 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. The two boys 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. 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. They both might get more screen time in the future. The only time they got a lot of screen time together was in the Sports Festival.

The two boys appear to be very comfortable around each other.

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. Since they both don't have a lot of screen time, the ship isn't that popular. However, people who ship it think it is really cute. 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.
 * Kosei likes to talk about romance with Sen and their classmate, Shihai.
 * In the Obstacle Course, Sen got 22nd place and Kosei got 23rd.
 * In the Ultra Analysis Book, Kosei notes that he feels comfortable just being around Sen.
 * The first time Sen smiles in the whole anime was when he was talking to Kosei in the Sports Festival (When they were teaming up together)