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. In the Sports Festival, they both teamed up together and Sen is seen smiling at Kosei. They both appear a lot together in the Sports Festival. A few moments later, they both teamed up with Neito Monoma and Shihai Kuroiro. Kosei and Sen 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.

In the manga, when class 1-A and class 1-B was hanging out, Sen and Kosei are both seen sitting together. Sen and Kosei are also both in the same internship, with Togaru Kamakiri. In volume 22 of the manga, it is stated that Kosei and Sen are close friends, Kosei being the only person who can occasionally make Sen smile when he is out of his stoic persona.

Because Sen and Kosei are background characters, they don't have a lot of screen time. However, when they have screen time, they are often seen together. 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. The two are very different in personality, as Kosei is very enthusiastic and competitive and Sen is very level-headed and introverted. They balance out each other nicely, as one has what the other lacks.

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 very cute that Kosei is the only person who can make Sen smile. Some people see the two as becoming 'more that friends'. Since they both don't have a lot of screen time, the ship isn't that popular.

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.