Gregory's ServerTIL the reason animes often reveal characters' blood types (heck, even Tuzki the rabbit has a canonical blood type even though rabbits have an ABH blood group system) is because in Japan there's a superstition that blood types correspond to personalities. Well, I guess that makes slightly more sense than my theory that the characters have blood types so you can include clinically accurate blood transfusions in your fanfictions.
More info:
https://anime.stackexchange.com/questions/3275/whats-the-significance-of-revealing-a-characters-blood-type
https://link.springer.com/chapter/10.1007/978-94-017-4453-9_24
OK, remember in your next fanfiction that you write that Mikasa Ackerman (blood type AB) from Attack on Titan ABSOLUTELY CANNOT give a blood transfusion to Tuzki the rabbit (blood type A) but it's safe for Tuzki to give a blood transfusion to Mikasa Ackerman (rabbit and human blood group differences aside). Wait no, they could get screwed over by the rhesus factor if it turns out they're A+ and AB-...