Mayuri Kurotsuchi is the Captain of the 12th Division, and 2nd President of the Shinigami Research Institute replacing Kisuke Urahara as he escaped to the real world for getting blamed for Aizen's hollowfication experiments.
Mayuri is a tall man with short blue hair and make-up. He wears an unusual sort of white hat which extends out to the right, and wears a large thick purple scarf around his neck, along with Shinigami robes and his captain's jacket. His middle finger on his right hand also has an abnormally large finger nail.
During the Hueco Mundo Arc, Mayuri changes his appearance a great deal, looking somewhat more Egyptian-ish, his ear attachments have become smaller and rounder, his teeth are now gold, his chin has a longer attachment, and he no longer wears his unusual white hat. Instead his hair is styled into "horns", which when looked at from the front looks much like a Pharaoh's crown, and his face make-up has changed.
Mayuri is a cold-hearted, arrogant short-tempered man, who is probably the only Captain that doesn't care at all about his subordinates, saying things like "I don't care if some of you die in the process" when he is out in battle, or searching for intruders in the Seretei. He also doesn't care about his Vice-Captain either, calling her "useless" most of the time. Despite his heartless nature, during the Hueco Mundo Arc, he seems to act sarcastic, pulling silly faces at his opponent, Szayel Aporro Granz, and also seems to tease Uryu Ishida, in a somewhat mean way.
Mayuri has a lot of useful abilities in battle, since he is a Scientist, he has created many unusual things to aid in his battles. He has even experimented on his own body countless times, so that if he becomes on the verge of death in battle, if he stabs himself with his Zanpakutō his body will turn into a green liquid which will prevent him from attacking, but will also protect him from harm. He also has a regenerative serum which he can use on himself if he looses a limb, so he can grow it back instantly. He also created a "Super-Human" drug that slows down a person's perception of time which he used on Szayel Aporro Granz to make 1 second appear like 100 years. He keeps this drug inside of his Vice-Captain, Nemu's body.
Aside from all his inventions, he has all the main powers of any Shinigami, and he has also experimented on his Zanpakutō several times to add features. His Zanpakutō's Shikai ability is that when an opponent is hit, it severs the brain signals controlling the neural impulses for movement in the limbs of the victim, regardless of where they were struck. Unlike regular paralysis, the pain receptors and nerve endings are unaffected by the weapon, so the victim can still feel pain, even in a paralyzed limb. His Bankai turns his Zanpakutō into a giant Caterpillar-like creature with what looks like a baby's head and arms. It produces a poisonous gas created from Mayuri's blood, and is dangerous to anyone who breathes it in except Mayuri and Nemu, and he also changes his poison each time he goes into battle so that no-one discovers an antidote for it. It also has the ability to extend numerous blades out from its chest in order to skewer opponents while they are concentrating on not breathing the poison. Mayuri's Zanpakutō also has a self-destruct action which Mayuri keeps in case his Bankai is turned against him somehow.
Not much is known about Mayuri's past, except that he was held prisoner in the "Maggots' Nest" and was also considered so dangerous that he was put into a single cell away from the other prisoners. He was then asked by Kisuke Urahara if he would like to be the Vice-Captain of the Shinigami Research Institute which he was planning on creating, and Mayuri accepted. 9 years later he also became the 3rd Seat of the 12th Division and when Urahara was forced to flee to the real world because he got blamed for Aizen's hollowfication experiments, Mayuri became the 2nd President of the Shinigami Research Institute, and also became Captain of the Gotei 13's 12th Division.