Duke is a witch and was Salem Saberhagen's underling during his plot to take over the world. Duke tries to stay on the straight and narrow after finishing his sentence.
Once Duke finishes his sentence as a cat, he needs a place to stay while he gets his life back in order. Hilda and Zelda offer to house him while he gets back on his feet. Duke wants to stay away from criminal activity from then on, but Salem is instantly a bad influence on him, persuading him to use his magic for his own gain. Duke's magic is incredibly rusty (since Witch Familiars have no access to their magic) and often goes wrong.
When Sabrina gets Duke to look after Hilda's clock shop, Salem tries to get Duke to conjure a voting booth that they can tamper with, but Duke's spell accidentally blows up the entire shop.
Feeling bad for getting her in trouble, Duke casts a spell on Sabrina to make her good at tap dancing (for an audition she has been preparing for unsuccessfully). Sabrina wakes up with two right-feet. Hilda and Zelda go into Duke's memory and find the spell so that they can reverse it.
Salem and Duke go to watch Sabrina and Harvey practice for their tap audition. Duke casts a spell to make Harvey a good dancer, but accidentally gives him the mentality of a child along with it. Sabrina unsuccessfully tries to chase down Harvey and goes to her audition alone. Duke joins her when Harvey is supposed to enter the dance and does the "Soft Shoe" with Sabrina, revealing himself to be a very good tap dancer.
Hilda and Zelda give Harvey a time out with the help of a tall, intimidating basket ball coach and Hilda manages to reverse the spell cast on him.
It is later seen that Hilda and Zelda have given Duke the job of teaching a tap-dancing class for the community.
Duke's magic manifests as a spiral of blue sparkles that shoot from his hand when he waves it. Duke casts all of his spells by incanting words that are meant to sound like Latin.