Kid is a tall and thin Shinigami boy of the appearance of a teenage boy. He is always dressed in a refined manner, mostly because of his status as a Shinigami. He limits his main wardrobe to a standard black suit with white rectangles, until later on in the manga series when he is seen wearing a white coat instead of a black one. The rectangles are placed in a manner that is completely symmetrical. There are at least six small rectangles along the seam where the sleeves attach to the jacket, and four small rectangles run along the front of his jacket. It is possible that these shapes could be replacements for buttons. His undershirt is a formal white dress shirt, and a metallic skull rests under his collar, and could be used as a tie. His color scheme is meant to copy his father's, where the only color variation between black and white is the color of his eyes. When going out on missions, Kid will typically cover himself in a cloak and mask that makes him look like a smaller version of his father. The reason behind this is unknown, but it could be speculated that it is to show his rank as the son of Death. Another speculation is that upon donning the mask on the opposite side of his Sanzu lines, it causes his hair to be perfectly symmetrical. Kid's most notable feature is the three white lines that cut across the left side of his hair, stopping in a completely straight line along the front and back of his head. They are the Sanzu Lines and they act as limiters to his power. When one or more line connects, his power increases exponentially and he becomes far stronger, and more dangerous. The lines, however, only connect when Kid is under the influence of Madness or determined enough. Kid greatly despises these lines because they throw off his physical symmetry. Mentioning these lines will send him into a fit, during which he curses his own existence, calling himself garbage. Kid also has a cowlick on the right side of his hair, but it appears he has not yet noticed it as he has made no effort to correct it. In the third episode of the anime, Kid's soul is shown as a red color and has the three white Sanzu lines, representing his asymmetrical bangs. Later in the show, however, his soul is shown to be blue with a skull shape and the three Sanzu lines marking him as a Shinigami. When under the influence of madness, five black lines appear over Kid's mouth. These give the appearance of his mouth being sewn shut. They also make Kid's face look like that of a skull. Another noticeable feature would be his eyes. Kid's eyes are two different shades of yellow, ringing around his pupil. The outside ring is a bright yellow while the inner ring is a dark yellow, similar to the color of gold. The reason behind this double-coloring is unknown, though many simply attribute it to the fact that he is a Shinigami. Also shown later in the series, it would seem to be like a scope, able to select out enemies and lock onto them.
While Kid is normally a force to be reckoned with, he has one major flaw that psychologically tortures him as well as physically handicapping him on occasions. He is a sufferer of a disorder which has become a trademark character quirk of his, called obsessive-compulsive personality disorder, or OCPD, which is manifested as an obsession over perfection. Objects and surroundings that are symmetrical can distract him from missions and Kid will often start to break down and undergo fits if he notices asymmetrical objects in the vicinity. Thus if anything is wrong by even the most minute of measurements (a crooked picture frame, not folding his toilet paper, 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 symmetrical appearance, but for the fact it divides out evenly. His obsession with symmetry is so profound such that he will refuse to fight in a battle unless everything is symmetrical, as he would be asymmetrical holding only one gun. On the other hand, if an enemy is symmetrical in appearance, he will refuse to fight it regardless less it upset its symmetry. The fact that the left half of his bangs is marked with three white stripes causes him much stress. As a result of his OCPD, Kid can be very hard on himself if the asymmetry of the stripes in his hair are pointed out by someone, calling himself "useless asymmetrical garbage".The Index of the Book of Eibon referred to this obsession as Madness of Order. He is usually very mature and precise in his ways. Additionaly, he is naturally calm and collected, and as such is perfectly fine with other people making their own decisions when it comes to symmetry or otherwise, even if he disagrees with their choices. However, there are times when his obsession with symmetry can greatly affect his calm personality and cause him to break down into childish fits. He doesn't appear to have any problem socializing and genuinely cares for his friends.