Belief that one’s blood type affects personality is common in Japan. Dating sites, celebrity profiles, and vital statistics for fictional characters often include blood type, and belief that it affects personal attitude or character is somewhat akin to belief in astrology in the United States. The beliefs have their roots in pseudoscience from before World War Two. In the 1970s a series of Japanese self-help books claimed that understanding blood type was the key to understanding personality, and a phenomenon was born.
It’s easy for an outsider to mock beliefs in pseudoscience like this, but humans do have a persistent desire to put themselves into boxes and groups, and to assign themselves certain group characteristics. This can take the form of astrological sign or blood type, but it also shows up in online quizzes, debates about which Ninja Turtle you are, or deciding which house you’d be sorted into if you went to Hogwarts.