Serirei

Serirei is the slash ship between Reigen Arataka and Serizawa Katsuya from the Mob Psycho 100 fandom.

Canon
Reigen hires Serizawa to work at Spirits and Such Consultation Office after Serizawa leaves Claw, an evil organization of espers bent on achieving world domination.

Fanon
Serirei gained traction as a popular ship in the spring of 2019, when Mob Psycho 100 Season Two was airing.

Even though Serizawa does not appear in the anime until episode 9, fans of the Mob Psycho 100 manga anticipated his debut and embraced his character as soon as he showed up on screen. Because of this, Reigen and Serizawa's few interactions this season were viewed as romantic by fans who know how their relationship develops in the manga.

Season Three (if there is one) should feature many more moments between Serizawa and Reigen that are present in the manga. As opposed to their interactions in Season Two, these are more romantically-coded and lend weight to the idea that Reigen and Serizawa both like each other.

Haircuts are a popular trope used in Serirei fanart and fanfiction. This stems from the fact that Reigen gives Serizawa a haircut on his first day of work, and this moment isn't actually shown in the manga/anime (giving fans room for interpretation and creativity). Drunkenness is also common in Serirei fanworks, since Reigen canonically cannot hold his liquor.

Fandom
Reigen Arataka/Serizawa Katsuya tag on AO3.

Serirei on Tumblr.

Trivia

 * Official Mob Psycho 100 art released in May of 2019 features Reigen and Serizawa in traditional Japanese wedding attire standing next to each other. They are each holding bouquets; in front of them are Mob and Ritsu, who are presumably the "flower boys" for the wedding. It's unclear what Studio BONES was insinuating with the art, but Serirei fans hurriedly embraced the picture as proof that Serizawa and Reigen finally got married.
 * A similar fan reaction happens for almost every official artwork where Serizawa and Reigen are standing next to each other.