Manny Calavera is the lead character in the last good Lucasarts game ever made, Grim Fandango. Manny works for the Department of the Dead as a combination of Grim Reaper and travel agent. When he uncovers a conspiracy that is depriving good souls of the afterlife they deserve, he abandons his job and sets out to get to the bottom of things.
His name and appearance are based on a Mexican tradition which states that the souls of the dead are allowed, for one day a year, to return to the land of the living and inhabit specially made dolls called calavera. This festival is known as the Day of the Dead, and plays a part in the opening areas of Grim Fandango.
For SC2K5, many fans of Guybrush Threepwood, another classic Adventure hero, decided that Guybrush's two previous losses meant it was time to give someone else a shot at becoming First Round Fodder. A lengthy campaign on Board 8, which involved convincing lots of people to either buy or 'acquire' Grim Fandango, was succesful in getting Manny in. Unfortunately, he drew a first round match against Solid Snake, and thus inevitably went out straight away. But hey, at least he lost to someone worthy.
He's also a character with a Wiki that is way too long for his popularity.