Blood type in Japan has a mythology of personality traits attached to it, much like Zodiac signs. This is the reason you often see character profiles that include blood type information. The following is a very condensed summary of the traits assigned to each blood type. Just as in real life, though, don't assume a character's personality will always fit.
Calm, obeys rules, values relationships with others, sensitive, deeply caring, nurturing, cautious.
Does things at his/her own pace, strong personality, optimistic, charismatic, adventurous, disdains the norm.
Strong sense of purpose, romantic, influenced by peers, orderly and good at organizing activities, very proud.
Well aware of his/her surrounding environment, values reason more than emotion, critical, (paradoxically) values the feelingsof others and tries to be useful to others, reluctant to take things to the fullest extent of their ability, highly intellectual.