Kid suffers from obsessive-compulsive disorder, or OCD, which is represented as an obsession about everything having to be perfectly symmetrical and takes the idea of perfection to the extremes. To him, anything symmetrical is beautiful and if it is off by any measurement (a picture frame off by millimeter, or a pose that is slightly out of place), he will try to correct it right away, even if the situation calls for utmost attention. His favorite number being 8 because of its symmetry. He is so obsessed with symmetry that Kid refuses to fight a battle unless he has both Patty and Liz together as they would not be symmetrical apart. The biggest problem Kid has is with his hair, because the left half of his bangs are marked with three white stripes. He has attempted to dye his hair all black to cover his asymmetric Sanzu Lines; however, due to his body being that of a Shinigami, it rejects the hair dye and soon returns to normal.
Showing Kid anything that is too asymmetrical or mentioning his hair will send him into a state of massive depression, which normally takes encouragement from Patti and Liz to break. As shown in episode 3, or Chapter 0.3, because the Pharaoh's coffin was symmetrical Kid wouldn't fight back, because he would ruin the symmetry. In the most extreme of cases, Kid will cough up blood and faint; this is usually set off by his hair being cut, or completely failing to make something symmetrical. If he sees anything perfect or symmetrical get broken or destroyed, or an opponent who is not symmetrical, it will send him into a blind rage (surprisingly similar to his father's previous violent personality) where he will unload all of his power to erase the offensive sight or cause of the destruction from his eyes. It is also because of his obsessive-compulsive personality disorder that Kid finds Liz and Patty's weapon forms much more appealing than their human forms. Because Kid was born a Shinigami it was never required that he train a weapon, and he was offered many partnerships in the form of arranged marriage although he rejected them all, purely because using any of them would break his symmetry.
However, his obsessive-compulsive outbursts aside, Kid is actually quite serious and sophisticated, and can be quite cool-headed in battle if the concept of symmetry doesn't enter his mind. Despite his prim and proper attitude, he does exhibit a few boyish characteristics from time to time, like when he is chasing after the runaway express in the desert, and he chooses to do skateboard tricks on the sand dunes whilst doing so, and he is quite happy to play basketball along with the rest of his team. He also cares for his friends, as he is seen expressing concern about Soul's scar at one point, and also threatens Gopher that he will never forgive him if he hurts Maka.
It is mentioned by Maka that it is normally hard to match soul wavelengths with more than one weapon, but Kid is able to do so because he, Liz, and Patti have a mutual admiration for each other. Kid admires Patti and Liz's positive souls due to his negative personality.
His father has mentioned that Kid is a shinigami and does not need to collect souls, but Kid says he wants to make a weapon of his own. 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 kishin eggs and two witch souls instead of 99 souls and one witch soul.