Stefan is Whitebeard's dog. It is a character invented by Oda when a fan asked him in an SBS if Whitebeard's mustache could be used as a boomerang. Oda answered that Whitebeard probably did whenever he took his dog, Stefan, for a walk.
Stefan is a white colored dog and bears a crescent-shaped white mustache similar to his master, Whitebeard.
Stefan, as Whitebeard’s dog, was loyal to him. He also enjoyed chasing and catching Whitebeard’s boomerang mustache.
Sometime in the past, Whitebeard took Stefan for a walk. During their walk, Stefan and Whitebeard played catch with the pirate's mustache.