Serirei

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

Canon
Serizawa and Reigen's first interaction occurs when Serizawa steps in front of a fiery psychic blast and saves Reigen's life. After the final battle between Mob and Toichiro, Reigen hires Serizawa to work at Spirits and Such Consultation Office. Serizawa officially leaves Claw, an evil organization of espers bent on achieving world domination.

Reigen and Serizawa become co-workers at Spirits and Such, and at some point Serizawa officially becomes deputy director of the company.

Reigen trusts Serizawa, counting on his guidance and support in many situations. Reigen often asks his co-worker "right, Serizawa?" when he's looking for reassurance, to which Serizawa usually responds with a simple "sure."

The Season 2 OVA "~A Journey that Mends the Heart and Heals the Soul~" takes place not long after the events of the season finale. The story emphasizes Serizawa's role as the "newbie" of Spirits and Such, so Serizawa is portrayed as slightly anxious around Reigen, desperate to please his boss. When Reigen sees Serizawa again after being stranded in an alternate dimension for a lengthy period of time, his first impulse is to tell Serizawa that he's "so happy [he] came back safe."

At one point in the manga, Reigen asks Serizawa out for drinks, saying: "Wanna go get a drink or something for once?" Serizawa politely declines, telling Reigen that he already has plans with some of his friends. Reigen responds with a disappointed "...I see." Many fans interpret this as manga artist ONE attempting to portray gay rejection, something (fans speculate) he also does with Terumob.

Despite Serizawa turning down Reigen's offer, there are many more moments in the manga that suggest that Reigen and Serizawa are interested in each other romantically. Reigen comments on Mob's crush Tsubomi, saying that their relationship is good because the two of them (Mob and Tsubomi) can "exist without distorting each other." With his back turned to Serizawa, he says, "I hope I can become a partner like that." Serizawa's expression is hard to read, but he seems surprised, maybe even curious. Both Serizawa and the audience cannot see Reigen's expression.

In the manga, there is a scene in which Reigen and Serizawa stand next to each other under Serizawa's umbrella. Reigen's hands are placed on Serizawa's shoulders. In Japan, this gesture can be considered an "Ai-Ai gasa." Ai-Ai gasa translates to "Love-Love umbrella" in English. Two people sharing an umbrella is considered a symbol of romance in Japan.

Reigen and Serizawa are both (presumed to be) gay-coded characters. Reigen asks Serizawa if there has ever been a girl he's liked. When Serizawa responds with "nope, none," Reigen is dumbfounded. Serizawa also wonders about Reigen's love life, noting that Reigen "seems so popular with the lady clients." The fact that Reigen never actually acts on any advances from these "lady clients" has led fans to infer that he might be gay. Reigen also spends most of the manga/anime hiding his "true identity" as a fraudulent psychic from the world; fans have drawn a parallel between this and the fact that many LGBT people feel the need to lie about their sexualities, believing that Reigen might actually be closeted.

Fanon
Serirei gained traction as a popular ship in the spring of 2019, when Mob Psycho 100 Season 2 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 3 should feature many more moments between Serizawa and Reigen that are present in the manga. As opposed to their interactions in Season 2, 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.

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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.
 * An interview for Animedia released in November of 2022 states that Reigen is "the senpai of Serizawa's life" and that Serizawa "holds respect and affection for Reigen." The term used in this interview, "敬愛する", literally translates to 'adore' or 'to respect and love'.