A doctor named Kimata Hara appears to be responsible for the introduction of the belief in Japan after he published an article in 1916 describing how he believed blood types affected character which would be further boosted by other high profile articles like Rin Hirano and Tomita Yashima’s "Blood Type Biological Related" which was published in the Medical Journal of the Army in 1926 and "The Study of Temperament Through Blood Type" by Takeji Furukawa, a professor at Tokyo Women's Teacher's School, which was published in the scholarly journal Psychological Research in 1927.

Furukawa’s article, in particular, become widely popular and introduced the concept of blood type personality theory into mainstream Japanese culture. Despite his lack of real qualifications, he even attracted government attention and was commissioned to lead studies aimed at breeding ideal soldiers. Following insurgencies in Japanese-occupied Taiwan, Furukawa did more research on blood types, finding that in his sampling of Taiwanese people there was a much higher percentage of type O individuals when compared to the Ainu, an ethnic group mostly concentrated in Hokkaido in northern Japan, who were considered “submissive”. Based on his findings, he suggested encouraging more intermarriage to reduce the number of type O Taiwanese.

Starting in the 1930s the belief began to fall out of favor as more research proved it to be unscientific, leading it to fade from both research and popular culture. That is until 1971 with the publishing of the book Ketsueki-gata de Wakaru Aisho (Understanding Affinity by Blood Type) by journalist and broadcaster Masahiko Nomi. In spite of Nomi not having any medical background, the book relying mostly on anecdotal evidence, and some heavy criticism from the Japanese psychology community, it proved to be a huge bestseller and is largely responsible for the blood type personality theory present in modern Japanese society. Nomi would go on to write almost a dozen books on the subject until his death in 1981 after which his son Toshitaka Nomi would continue his work and establish the Human Science ABO Center in 2004.

Although removed from its uncomfortable connections to ethnicity, stereotypes associated with different blood types still persist in Japan, as well as Korea, Taiwan, and other parts of Asia, feeling very similar to how astrology is viewed in America and parts of Europe, even having “blood type horoscopes” regularly featured in daytime TV shows and newspapers. Supposedly, there have been modern instances of workplace, social, and educational discrimination based on blood type but weren’t able to find any solid reports of this. In general, it seems that many people in Japan think that blood type may influence personality but is not what determines it and the superstition mostly manifests itself in fad diets and training regimes based on blood types, the aforementioned horoscopes, and in things like game and anime character descriptions. It was, however, the central subject of a short 2013 ONA series called Ketsuekigata-kun! which was based on a Korean 4-panel webtoon (a type of comic) known as A Simple Thinking About Blood Type.

Some traits supposedly associated with the blood types include:

Type A: Calm, Responsible, Introverted, Patient, Overcautious, Uptight
Type B: Creative, Flexible, Passionate, Optimistic, Forgetful, Self-centered
Type AB: Rational, Empathetic, Organized, Adaptable, Aloof, Unforgiving
Type O: Outgoing, Social, Ambitious, Natural Leader, Arrogant, Jealous
*Note: Almost all Japanese people are Rh positive so it does not factor into this