A person who states the obvious. So rather than let the subject on everyones mind go, he or she must "state it." Stating, the result of being a stater, is annoying. The stater doesnt really want to be the person to be quiet, rather the stater wants to point out the obvious hoping to get some attention.
Optimistically, it can be a friendly reminder for his or her friends. (see example)
Depending on the situation, a stater can either be a good or bad.
Originated from the word hater
Malvchild: Yo man, your girl totally has a moustache.
Garry: Way to be a stater.