Bloody Baron

From the Encyclopedia of Speculative Fiction - http://encyclopedia.wizards.pro

The Bloody Baron is one of the ghosts that haunts Hogwarts School of Witchcraft and Wizardry. His domain is Slytherin House. He is the only person, living or dead, who can exert any control over the poltergeist Peeves, given how Peeves is terrified of him.

The Baron is covered with silvery blood stains, which has never been explained in the books but theorised by some fans to have been Unicorn's blood, which is likewise silvery in colour; however, given how all ghosts of Hogwarts have a silvery hue this may be coincidence.