Anti-William was born in Anti-Fairy world when Foop was about 13 and Anti-Rosemary was about 9 (Anti-Fairy years). Unlike most Anti-Fairies, he was born without the ability of speech, and was born abnormally small. As soon as they met, his Fairy counterpart often liked to tease and make fun of him due to his speech problem and his size. His older sister is extremely overprotective of him to the point where he may get annoyed with her, while his older brother, Foop, prefers to distance himself from both of his siblings.
Because of his sister's extreme overprotectiveness, he likes to explore whenever he gets the chance and is curious about new things.