In anime it isn't uncommon for a character's blood type to be included in their profile. Why? Because in Japan blood type is believed to influence your personality. Here is a list of the blood types and the personality traits they represent.
This type is calm, obedient, sensitive, cautious, serious, careful, and often values their interpersonal relationships. They are often polite, studious and disciplined, and tend to be empathic and self-sacrificing. This personality type will often be indecisive, introverted, nervous, and detached, and may have a tendency to be worrisome and hesitant to express their feelings.
Examples: Matsudaria, Momiji.
This type tends to have a strong personality and is usually optimistic, adventurous, and easy-going. This personality is usually outgoing, friendly, sensitive and kind, but has an inclination to be forgetful, disorganized, noisy and spontaneous. Type Bs may also be indecisive and prone to exaggeration.
Examples: Koume, Kunakida, Sugishita.
The Type AB personality is often a very critical personality, but they try to be useful and are often very aware of their environments. They are diplomatic, proud, efficient and quick learners, but they tend to be delicate, short-tempered and dependent. This type may also be moody or brooding, and maybe a bit too serious about themselves.
Example: Takeuchi.
This type is romantic, proud and organized with a strong sense of purpose. Warriors tend to be confident, judgemental, strong-willed and dedicated. They also have an inclination to be over-confident, self-centered, stubborn, insecure and, often, workaholics.
Example: Yaegashi.
For more information on anime blood types, visit Anime Blood.