In Japan, one's bloodtype is viewed much like one's zodiac sign in the United States. Certain bloodtypes have certain persnality characteristics, and the people with those bloodtypes are more likely to be compatible with certain other bloodtypes.
People with A blood types are most likely to be perfectionists who are not very social. They do not like to publicly dissent from popular opinion, even though they may privately disagree. Mistakes stress them out, and they keep to themselves and make a living being a "brain" (hmm, sounds like one waterbased soldier we know ^_-). These people would probably be a novelist, scientist, or doctor. Type A is the most common type in Japan.
B types love to stand out. They enjoy helping others and are often outspoken and funny. B's will put up a spirited fight if someone says something they disagree with, and any mistakes made will be corrected almost immediately before anyone else may notice. This blood type is common among leaders and people in the television industry.
AB people are the unsual people (Hotachan maybe? ^.^) as AB is the rarest blood type around. These people enjoy creating their own way of accomplishing things and will perform tasks on their own terms.
As type O's are universal donors, they get along with everyone. O's will change their opinions to go with the popular mindset and don't really care about mistakes, but trendiness is extremely important to them. Most O's are artists or in the television industry. O is the most common type in the U.S.