King Boo

King Boo is the leader of the Boos, ally of Bowser, and nemesis of Luigi. When Professor E. Gadd captured one of his underlings, Boolossus, King Boo retaliated by attacking E. Gadd's laboratory, freeing all the ghosts he had captured for his research, and somehow erecting a haunted mansion nearby. He then tricked Mario and Luigi into coming to the mansion by sending them a letter saying they'd won it in a contest; King Boo and his ghost army were able to capture Mario this way, but Luigi put up more of a fight thanks to E. Gadd's Poltergust 3000, and he was able to defeat King Boo and save his brother, which somehow also made the mansion un-erect itself. This was the first in a pretty long series of conflicts between the Mario Bros. and King Boo, but King Boo has never ended up winning (and curiously enough, he's never been able to defeat Luigi like he has Mario). On an unrelated note, he may be friends with Petey Piranha.