"Symmetry: The quality of being made up of exactly similar parts facing each other or around an axis. Or in other words, perfection"
Kid suffers from obsessive-compulsive disorder, or OCD, which is manifested as an obsession over symmetry, otherwise known as asymmetriphobia. Things (and people) must be perfectly symmetrical, lest something unknown and awful happen. Thus if anything is off by any measurement (a picture frame off by millimeter, or a pose that is slightly out of place, for example), he will try to correct it right away, even at the expense of the mission at hand. His favorite number is 8, not only for its symmetry, but for the fact it divides out evenly. He is so obsessed with symmetry that Kid refuses to fight a battle unless he has both Patti and Liz together, as he would be asymmetrical holding just one. The fact that the left half of his bangs is marked with three white stripes causes him much stress. He has attempted to dye his hair all black to cover these asymmetric Sanzu Lines; however, due to his body being that of a Shinigami, it rejects the hair dye and soon returns to normal.
He is usually very mature and precise in his ways. Kid is perfectly fine with other people making their own decisions, even if he disagrees with their choices. However, there are times when his obsession or just a miscalculation can greatly affect his usual calm self and cause him to become quite childish.
His father has mentioned that Kid is a Shinigami ("death god") and does not need to collect souls, but Kid says he wants to make a weapon of his own specifications. His father also says that since he uses two weapons and the guns do not count as a set, he will have to collect twice as many souls: 198 evil human souls and two Witch souls instead of 99 souls and one witch soul. However, he seems perfectly fine with this.
Liz and Patty; the twin pistols
Death the Kid, just Kid to his friends, is the son of Shinigami, also known as Death (the Grim Reaper of Western legend.) Because of this, he is also considered a death god himself, making his natural abilities outshine those of his fellow students. Not much is known about his past in general, other than he was raised in a large mansion and carried out the work of destroying corrupt souls.
Sometime during his past, he heard the rumor of the Thompson sisters, two criminal sisters in Brooklyn who made a living off of mugging and similar criminal activities. More to Kid's interest, they both possessed weapon blood that allowed them to turn into a matching pair of .38 caliber pistols. He tracked them down and convinced them to become his weapons for the sake of symmetry. It's said that, as a Shinigami, he doesn't need to make his own weapons, but Kid insists upon doing it himself.