What's the significance of revealing a character's blood type?

Question

A lot of Anime shows will reveal the blood type of one or more of the main characters:

• What's the significance of it? What purpose does it serve?
• Where did it come from?

Answer 1

Blood type in Japan is associated with temperance or personality. On Facebook in Japan, as well as Wikipedia and matchmaking sites, blood type can be listed. Many horoscopes and matchmaking services also use blood type to match people together.

According to Wikipedia:

It is common among anime and manga authors to mention their character's blood types, and to give their characters corresponding blood types to match their personalities. Some video game characters also have known blood types. In addition, it is common for video game series to allow for blood type as an option in their creation modes.

The associated personality traits (from the chart here):

• A
  • Better traits: Earnest, creative, sensible, reserved, patient, responsible
  • Worse traits: Fastidious, overearnest, stubborn, tense
• B
  • Better traits: Wild, active, doer, creative, passionate, strong
  • Worse traits: Selfish, irresponsible, unforgiving, erratic
• AB
  • Better traits: Cool, controlled, rational, sociable, adaptable
  • Worse traits: Critical, indecisive, forgetful, irresponsible, "split personality"
• O
  • Better traits: Confident, self-determined, optimistic, strong-willed, intuitive
  • Worse traits: Self-centered, cold, doubtful, unpredictable, "workaholic"

Answer 2

Overview

Blood type carries a significant weight in Japanese culture, so much so that Japanese are often surprised when people of other origins are not familiar with their blood type. In their culture, they associate every blood type with a certain personality and demeanor.

There is an interesting history behind blood types; in 1900, A, B, and O were initially pinned down; more than twenty years later, in multiple reports over a number of years, there were links made between specific racial traits and blood type (such as the rebelliousness of the Taiwanese, who have a majority of O-type). As a result, and ever since, the blood type of people has been indicative of other traits as well.

Relationships

Even now-a-days, blood types are uesd in Japanese culture. This can go to such an extent that it's common for young women to select husbands based on it. There are even compatibility suggestions, which I've put in a chart below:

(Compiled from JapanToday's article^[1])

Traits

The personality traits that are determined by a blood type are usually consistent between negative and positive blood type. (I focused mostly on the positive traits; @kuwaly mentioned, some more negative ones.) Some of the more dominant traits are^[1]:

Type A

• Cautious
• Empathetic
• Kind
• Good host
• Quiet (to prevent arguments)
• Honor student material
• Big on cleanliness
• Hard working

Type B

• Independent
• Agressive
• Optimistic
• Friendly and open
• Humble
• Afraid of being alone
• Flexible thinker
• Like to play

Type AB

• Dream chasers
• Don't desire success as much
• Strong spiritually
• Calm and rational
• Sensitive and easily hurt
• Private life is important
• Pursue a variety of interests
• Have unique ideas and are creative

Type O

• Realistic
• Vigorous at earning a living
• Strong in face of adversity
• Romanticists
• Ambitious
• Very cautious
• Don't care about small things
• Devoted

The article I've been sourcing has even more on this, including recommended diets and exercises for the various blood types.

Why assign one to a character?

To the original question: Why assign a blood type to a character? Well, you can almost think of it like assigning an astrological sign or even a body build; these characteristics tend to define a character as much as the blood type does (in Japanese media). Not only does it become part of that character's identity, but it helps to define them both to the creators and to the viewers.