https://www.cbr.com/anime-blood-types-explained/
There are numerous ways we use to assess human personality. Astrology is one of the most common methods, where people use planets and stars based on birth dates to evaluate individual personalities. Another is aura, where colors serve as a means to identify someone.

However, the Japanese opt for a different approach - blood types. Indeed, in the popular anime works of the Land of the Rising Sun, filmmakers often employ blood types to depict human personalities. A classic example is in Lucky Star, where characters have gathered to discuss blood types much like how we discuss zodiac signs.
According to statistics from motto-japan , the use of blood types to determine personality traits began around 1916, based on research by Dr. Kimata Hara. Though it only gained real prominence around 1925. Researcher Takeji Furukawa presented his personal study on whether schools' entrance exams should evaluate students' personalities. This hypothesis gained popularity until the 1970s when Masahiko Nomi, a supporter of Furukawa's theory, published their own book on blood types determining personalities.

As per this, blood type A is meant to describe individuals who are extremely organized, neat, clean, shy, and reliable. They are introverted, enjoy doing things for others but are prone to stress. They are also the ones people turn to for advice or mediation. It could be said that blood type A personalities are good mediators and 'valuable mediators.' Ami Mizuno, the Sailor Mercury, is a prime example of this blood type.

On the other hand, blood type B individuals are creative, curious, eager learners, but they break rules in doing so. They can adapt and think outside the box. They are also unpredictable and prone to dropping everything to pursue their interests. Because of this, they are seen as irresponsible, adventurous, and somewhat impulsive. My Hero Academia's beloved homeroom teacher, Shota Aizawa, belongs to this category.

Next is blood type AB, which is a combination of the two previous personalities, embodying aspects of both. They are considered odd, aloof, and hard to read. This blood type is not common in Japan, which makes them stand out even more. They possess all the strengths of being outgoing and eager learners, but they also tend to hesitate and be unstable. Mikasa Ackerman in Attack on Titan is an example with blood type AB.

The last group is blood type O, seen as natural-born leaders and 'carriers and general doers,' no matter what happens. Blood type O individuals are very rough around the edges, but they are not malicious. These are people with leadership qualities who even work in crucial positions. They are very optimistic and tend to see the positive aspects of others, but they struggle to express themselves. Nico Robin in One Piece is a typical blood type O individual.