Mister Nailbrain is a prinny, the ditzy and awkward sidekick of the Demon Sisters. He obtained his name from having his brain blown up in the Infernal War. Fortunately, 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 his 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 Infernal War, where the forces of Laharl's demonic horde 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. There was only one problem. He was a buffoon.