“Roderick Kilroy” is a fanmade name for the ghost of a human man who haunts the Haunted Mansion, identified in blueprints only as a "small ‘Thing’".
A member of the Over Four Hundred Club, cranky-looking Roderick Kilroy (who sometimes covers his wild mop of hair with a bowler hat) can be seen peaking from beneath the banquet table while the other ghosts frolic.
Behind the scenes
Roderick Kilroy, who debuted when Disneyland's Haunted Mansion opened in August of 1969, was originally drawn as a little ghost with only very blurred human features and no hair at all; he was inspired by the Thing as it appeared in Charles Addams's Addams Family drawings (as opposed to the disembodied hand the Thing was depicted as in the TV series and films). In the ride itself, he was redesigned as a more human-like figure. Whether or not the ride's figure wears a bowler hat has varied over the years and versions.
The character is nameless in the ride and all official spin-off media. In 2016, Emma Mosier opined that his name ought to be “Roderick”, while Dan Olson of Long-Forgotten Haunted Mansion favors “Kilroy”, in allusion to the notorious meme, names which have later been rejoined into “Roderick Kilroy”. Interestingly, Mosier also speculated that the character may be related to the Organist due to the physical resemblance between them.