Don Diego is Count Duckula's cousin who first appeared in the Season 1, Episode 2 episode, 'Vampire Vacation'. He is described by Igor in that episode as a "pyromaniac", someone who likes setting fire to things. Although Don Diego thinks he is "good and kind and wonderful", the villagers believe he is "dastardly" and evil.
- In the episode Vampire Vacation (Season 1, Episode 2), one of the Spanish locals presents Don Diego as his "honorable highness, the dirty, diabolical, dastardly and disgusting, DON DIEGO!!!" The repeated sound of the letter "d" in this case is called alliteration in literary studies. In this episode Don Diego sets fire to Duckula's castle. Don Diego also reveals to Duckula that he chopped off all of the villagers except "the really fat ones", which he had been saving for that particular night, after which there would be "bingo to follow." Duckula refuses to take part in Don Diego's dastardly schemes and after an argument Don Diego only agrees to spare the villagers' lives if Duckula faces the famous bull, El Loco, in a classical bullfighting arena.