Digby Sellers (aka Mr. Digg) is Lizzie's cool, laid-back substitute teacher. In his lessons he implied that he had educated celebrities like Frankie Muniz and Christina Aguilera. He befriended Sam McGuire in one of the episodes and dated Matt's teacher, Miss Chapman. In Lizzie's view, Mr. Digg actually gave great advice and taught stuff that was not only true but also very useful, even when it wasn't in the lesson plan. Plus, Mr. Digg also had his own unconventional way of teaching subjects, and didn't believe in using a lesson plan.