Despite having "Duck" in his name, Duckworth is actually an anthropomorphic dog. He worked for Scrooge even before Huey, Dewey and Louie moved in. He's Scrooge's chauffeur and errand boy on occasion, and used to be Scrooge's only household servant until the hiring of Mrs. Beakley.
He usually appears as a supporting character, and his notable roles are in "Duckworth's Revolt" and "Take Me Out of the Ballgame". In these two episodes he shows he's a capable leader and his out of the box thinking comes in handy, whether its helping Doofus hit a baseball or steering an out of control flying saucer.
As a jack-of-all-trades of McDuck Manor, he is happiest when he's helping someone and has no real interest of going out in the world to seek his own fortune.