Blood type
Ketsueki-gata is a popular cultural belief system in Japan, wherein your blood type signifies specific personality traits - similar to the western zodiac. This is why you'll often see blood types mentioned in other Japanese games. The belief in ketsueki-gata persists without scientific backing.
Type O
People with type O blood are generally considered confident, determined, optimistic, outgoing, easygoing, and energetic. They are often labeled as natural leaders. On the flip side, they are often percieved as being arrogant, selfish, and lacking attention to detail.
Type Os are generally believed to get along well others, excluding AB personalities. Type As often percieve type Os as selfish.