Frank Ockenfels / The WB
The ghost: "Buffy the Vampire Slayer" and its spinoff "Angel" are brimming with all sorts of beasties, ghoulies and ghosties, and several main characters straddle the line between life and death. But the most memorable ghost is, perhaps, Dennis Pearson, a spook who, when given the chance, turns out to be as helpful and loyal as he is eerie. In flashbacks, he's portrayed by BJ Porter, but most of the time, he's invisible and voiceless.
Boo who? Dennis haunts the apartment where he lived with his mother, Maude, and died by her hand. In life, Maude learned Dennis was moving out to be with his girlfriend and built him into a brick wall, then subsequently died of a heart attack. Cordelia Chase, played by Charisma Carpenter (above left, with David Boreanaz and Glenn Quinn), moves in, battles Maude's ghost and makes peace with the ghost of Dennis, her new invisible roommate. Dennis proves benevolent and helps Cordy around the house. He's extremely loyal to and protective of Cordelia, who mentions that he's "a ghost who's good with the loofah."