Duke is a red and black striped frog and Chocolat's magical familiar. As her familiar Duke is supposed to try his best to help her collect as many hearts as possible, but you mostly see him lounging around on her bed or sleeping. Instead of giving Chocolat advice, he makes discouraging comments. He finishes all of his sentences with '-kero' which is similar to 'ribbit' in English. He fights a lot with Blanca and often calls her a rat.
In the manga
Duke is really Poivre, Cinnamon's brother won turned himself into a familiar trying to hid since he was searched by the palace guards for hiding Cinnamon, but the spell backfired and he turned into a real familiar. He got back to being a human after getting a white heart.