Duke Onkled is the ruler of Gamelon island and a cousin of King Harkinian in Zelda: The Wand of Gamelon.
The King risks his life to save Duke Onkled when Gamelon is under attack by the evil forces of Ganon, only for Zelda to later find out that Onkled is corrupt and allowed Ganon to take over his land and imprison many people, including the King. Luckily, the King is saved and punishes the Duke by making him scrub all the floors in Hyrule.
Duke Onkled, being a one-dimensional character like the others in the Zelda CDi games, is characterized exclusively by cowardice. He is selfish and only out for his own gain.