Mortimer Spellman is Sabrina Spellman's cousin.
Mortimer was once on his way to being a very skilled witch until he decided to become a witchgician - a witch who decides to become a magician. By doing this, he made himself a sort of black sheep of the family.
Mortimer visits his younger cousin Sabrina to discuss the family secret and she gives him the confidence to perform at her boyfriend Harvey's little brother's birthday party. Unfortunately, Mortimer sends the boy to another country. He eventually is able to bring him back.
Mortimer then leaves Sabrina with a clue to the family secret - a small cardboard superhero with an ABC printed on his chest - a reference to the Spellman family.
Mortimer is later a guest at Sabrina's wedding four years later.