Mister Nailbrain is the clumsy, accident-prone sidekick of the Demon Sisters. He obtained his name from having his head blown up and nailed back together during the Hellmouth Conflict. He was saved by demonic doctors, who gave him a new brain, which was nailed to his skull, and his head stitched back together.
Unfortunately for him, this new brain caused him to become a foolish and clumsy prinny who was rejected by most demon officers. Eventually, he found a place in the Secret Police, and was assigned to serve the Sisters. Though he constantly messes up, he is unfailingly loyal to his masters, and strives each time to get the job right, each time getting better. A little.
Mr. Nailbrain was actually a feared and dangerous prinny leader in his past life. He fought in the Hellmouth Conflict, where Laharl's demonic armies poured onto the surface of Earth. However, he was shot in the head and killed in combat. In attempt to restore the great leader, demon doctors brought him back to life.