From AV Club:
The folks at OutsideXBox have shed some light on why Japanese game developers thought such information was important. In Japan, it’s believed that a person’s ABO blood group is an indicator of their personality. People with blood type A, like Chun Li, are marked as hardworking but stubborn, while people with blood type O, like Street Fighter’s Guile or Resident Evil villain Albert Wesker, are considered ambitious and confident, but also arrogant and vain. The notion was popularized in the ’70s with a book on the subject by author Masahiko Nomi, and has remained in vogue ever since. Like the idea that a individual’s personality is affected by their astrological sign, there’s little in the way of scientific evidence to back the belief up, but it does provide some insight into why, for example, developers assigned a blood type of AB to Final Fantasy VII’s Cloud Strife, since that indicates he’s controlled and rational, yet aloof and indecisive.