Twelve bears no active personality, usually copying his opponents' quips and uttering "target" and "complete". Beyond this, Twelve is only capable of expressing himself in a binary code cypher (00001 = A, 00010 = B, etc.). His win quotes imply a simplistic, aggressive and even thuggish personality, akin to that of an arrogant foot soldier.
In his rival battle against Necro however, Twelve is shown speaking in a normal manner. Twelve isn't without quirks in his personality, however. In one of his win poses he looks around to make sure he isn't being seen, jumps into the air, transforms his hands into wings and flies away while shouting 'woohoo!'.