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The developers used blood type to tell the characters’ story, their traits and how they interact with the world.
Aerith’s blood type is O. People with blood type O are believed to be outgoing, energetic and social. Aerith is upbeat and joyful, and several times in Final Fantasy VII attempts to cheer up her fellow party members. She’s the game’s resident cheerleader!
At their worst, they’re (blood type O personalities) self-centered, cold, doubtful, unpredictable, and tend to be “workaholic”. Like the time when she took it upon herself to run off to the Lost Capital herself.
Also, consider Aerith’s donor status. People who have blood type O are “universal donors”; they can donate to all other blood types. This is perfect for [Aerith’s self-sacrificing nature]. Also, a person with type O can only receive a transfusion from someone with blood type O. This hints that Aerith is the last of her kind.
Barret, on the other hand, has a gun.
Cloud is blood type AB. People with this blood type are “hard to categorize”. They can be:
both shy and outgoing. They easily switch from one opposite to another. AB people are trustworthy and responsible, but can’t handle it when too much is asked of them. They don’t mind doing favors or helping out, as long as its on their own conditions.
This fits Cloud, has he doesn’t really have a stable personality. He’s “arrogant and proud” at first, but only because he’s pretending to be a SOLDIER.
As blood type O are known as “universal donor”, AB types are “universal recipients”. Cloud’s type can receive from all other types. Who Cloud believes he is happens to be an amalgamate of other personalities. But who isn’t like that? Note that due to being the main protagonist, he’s also somewhat of an everyman.
Also, note that type B can receive blood from AB.
Original post: Blood type personality in Final Fantasy