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. 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 Bakugou, 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 were 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. 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
Since Kosei and Sen don't have a lot of screen time, and usually are only seen in the background, the ship isn't very popular. However, Kosen is surprisingly the most popular ship for both characters. On AO3, KoSen is the most written ship for both Kosei and Sen.

The ship gained its supporters when people began to realize that the two are rarely seen without one another, and grew in popularity by a bit when it was revealed that Kosei and Sen are best friends.

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 anime was when he was talking to Kosei at the Sports Festival.
 * In the My Hero Academia fifth season trailer, Kosei and Sen are seen standing next to each other when class 1-B arrives to the Joint Training Battle.