Derek "Del" Uglio is Billy's long lost twin brother, despite the fact that they look nothing alike or are even the same age, because they both have a birthmark that combines to create a map to an ancient treasure. His name is supposed to translate into "The Ugly", but the correct Spanish for it would be, "El Feo". He is a Mexican wrestler with a high temper and is considered the ugliest man on Earth. He calls Billy "hermano", which is Spanish for "brother".
In truth, he is actually incredibly handsome, however he and everyone who sees his face considers him ugly for whatever reason. At one point, Harold lampshades how Del Uglio is his son when he says: "...and I got the son I never knew I had because it doesn't even seem possible".
He appears in the episode "El Dia De Los Muertos Estupidos". Del Uglio reappears in another episode, but has never reappeared on the show again, nor was he referenced as Billy's brother. This was most likely due to his debut being the second-to-final episode.