Wikipedia has a page on the idea that blood type predicts personality, a discredited theory that nevertheless remains widespread in Japanese and Korean popular culture.
The idea seems to hold a similar cultural position as star signs and astrology and is used as a platform for discussing relationship compatibility and vague personality characteristics.
This is from a 1998 article from the Journal of Cross Cultural Psychology:
For example, in Southeast Asia, one can find many ideas about personality ranging from typologies based on blood types, horoscopes and fortune telling of various kinds, to personality theories similar to the “Big Five“. For example, since World War II, it has been popular in both Japan and Korea to classify people according to their blood types and elaborate theories of the qualities and characteristics of people with type A, B, AB, or O blood. Type A people, for instance, are considered to be serious and ordinary and type Bs to be unique and “going my own way.” Young Japanese commonly exchange this information about themselves and make predictions about the course of their relationships based on this information.
The Wikipedia notes that many Japanese video games often list the blood types of the characters, presumably to reflect their presumed personality.