There is no scientific basis for the idea that blood types can give a person insight into her personality, but nonetheless this has been a common cultural idea in Japan for many years. Newer anime fans are always bewildered, for example, by the fact that “data” sheets for their favorite anime girls include blood type alongside such traits as bust, waist, and hips. Due to the commonality of blood typing, the Anime-Manga Roleplaying Network has a blood type section on its character sheet.
However, in Japan, blood typing is, like, totally 1984:
Broadcasting Ethics and Program Improvement Organization, a third-party organization formed by NHK and other broadcasters, has received more than 50 complaints from viewers about programs featuring characterizations based on blood types.
The last paragraph of the article was interesting to me, because while I knew that blood typing had no scientific basis, I didn’t realize how dated the process was:
Tatsuya Sato, assistant professor on social psychology at Ritsumeikan University said that characterization by blood types in Japan was extremely popular in the 1980s, but the boom subsided after researchers failed to uncover scientific evidence to characterize people based on their blood types.
The practice hasn’t disappeared from anime, though; we know, for example, that Echizen Ryoma is type O and Uzumaki Naruto is type B. (It may be that blood types are used with fictional characters to assist viewers in understanding their personalities. Similar to saying something like “he was on the football team” here in the U.S., it’s a gross generalization that is immediately understood.)