William is physically a 10 year old vampire, although it's unknown how old he really is but he seems to have been involved with Ms. Bitters in the past. He likes adventures and traveling the world, and uses his vampire-like abilities to aid him on his adventures. Most recently he overthrew reptilians in Mexico thanks to Dib, Zim, GIR, skool children and Mexican gangsters. Despite his heroic nature, noble heart and loyalty to his friends he is also somewhat lazy and gets stressed (at the end of the reptilian defeat he warns the Mexicans he lead to victory to stay away from him). After the defeat of the reptilians he aids Dib and his comrades onto further adventures. He rarely ever fights, but it can be justified as most of his enemies are dead before they can fight back. He later joins Dib and The Resisty in overthrowing the Almighty Tallest. He declares that his main enemies are the tyrants and dictators he helped overthrow.
He was born in a small western town. He has forgotten many details of his mortal life but he did remember his family, friends and hometown (all of the names forgotten), the fact that he was attacked by a horde of vampires, and he was the only survivor after he was bitten and left for dead by their unknown leader. Despite still being physically a minor, he is shown to have fought in the Civil War, World Wars One and Two, and the Cold War, until he left his military career and focused on fighting for poor villages or third world countries. His favorite instrument is the guitar which he uses to play "theme songs" for characters (for example Dib's theme is "God's Gonna Cut You Down" by Johnny Cash). His main love interest is Zita who is the only girl besides Gaz and Tak that has the most interactions with him. While he joined Dib's classmates and The Resisty, he agrees to help them if they help him avenge the deaths of his childhood by hunting down the vampire who killed his family and friends, and the horde that destroyed his village and possibly cure him. He is quick to learn has he is naturally good at manipulating recently invented technology. He later discovers that the vampire that bitten him was a female named Eve, who was ordered to whip out his town by her clan leader, who William still plans on getting even with.
William is portrayed as a heart throbbing hero. He has fought in nearly every American war before he became a revolutionary. He also can be hateful to those who annoyed him often telling former brothers in arms that caused him stress to stay away from him or he'll tear them a new one. He is best friends with Dib after they met each other and Dib is the one who set him up with Zita. Although William is clearly a romantic it is proving he was an abusive lover in the past saying that the mess in his home is a constant reminder of what happens when he hit his girlfriend. He also has a sense of humor. He has a love for babies. All in all he is still true to his quest for vengeance against Eve's clan leader despite being friends with her.
- William appears to be based on Edward Cullen and Stefan Salvatore.
- He owns an IPod which reveals his favorite songs are Where the Hood at by DMX, Pony by Ginuwine, Striat Outta Compton by NWA, Salt Shaker by the Ying Yang twins, and Get Back by Ludacris.