As a fan of character relationship dynamics, I'm seeing a lot of odd "Serizawa and Reigen's relationship has no weight or substance" discussion on my tl recently, and it's important to me that this fandom actually understands the connection between these two characters.
The most prominent and probably most important narrative parallel being how Toichiro "freed" Serizawa by having him depend on him and CLAW, whereas Reigen freed Serizawa by encouraging him to be his own person while still including him in S&S and promoting real healthy connections with other people.
Toichiro towering over Serizawa while Reigen kneels to be at his level... Reigen sees Serizawa as an equal. Despite technically being "his boss," he includes him and encourages genuine discussions in both a personal and workplace environment.
This, and the way the two of them seem equally amazed and supportive/reliant of the other. Serizawa in Reigen's logical knowledge and street smarts and Reigen in Serizawa's psychic powers and spiritual experience.
On a more wholesome and personal note, there's also the fact that, after the climax of the finale, Serizawa was definitely calling Reigen in a panic to find out if he was alright.
Long story short, there's a lot of character interactions and development of these characters and their relationship if you really pay attention! Sure, they both have their own lives (Serizawa going to night school and Reigen... doing what Reigen does), but it's not like they're "just coworkers." They're very close, and they have a lot of canon material to work with!
