Blood Type.

In Japan it is believed that Blood Type has an influence on a person's personality compatibility and characteristics, which is why so many manga have the blood type of the main characters noted. Rei's blood type is listed as AB.

People with a blood type of AB are said to be cool and controlled people. They are rational and may appear standoffish, but remain popular nonetheless. They are known to potentially be critical and undeceive.

Clearly Sailor Mars' blood type fits her to a T. Especially in her manga incarnation where next to nothing seems wrong. Of course, her anime version could hardly be called cool and controlled, but this is so true of her manga form that a counterbalance is found.