Irwin is, next to Mandy and Grim, Billy's closest friend. He is a nerd who desperately aspires to be cool and frequently ends sentences with "yo". He has an unhealthy crush on Mandy (ever since Billy possessed Mandy in Get Out of My Head and said to him, "You are such a cutie!"), and longs to gain her affection, often stalking her. He frequently fails in both those goals, typically embarrassing himself and getting beat up by her. Irwin might be one of Billy's closest friends, but is often forgotten in dangerous situations, as well as being used as bait. Irwin's mother is a mummy, which make Irwin 1/2 mummy, and Irwin's father is a dhampir, (Irwin's paternal grandfather is Dracula,) which makes Irwin 1/4 vampire. Because of Billy putting his life in danger almost every day, it's natural for him to have a cowardly personality. It is possible because of his mostly supernatural heritage that he was born evil. However his father Dick taught him about being a good person (which is fortunate since he would have become a male version of Mandy: selfish, cruel, merciless and so on). But like his chances of getting a date with his dream girl, the chances of his evil personality are slim to none. Irwin has a habit of adding "yo" to the end of his sentences.

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