Duke Williams is a native of Topeka, Kansas and a muscular surfer from Waimea Beach, Hawaii who ends up on the island after riding a tsunami out to sea and getting lost. He's a bit of a womanizer, overtly flirting with both Ginger and Mary Ann and to a lesser extent, becoming friends with Gilligan who looks at him as a hero. More than resolved to stay on the island with the girls, Mrs. Howell and the Skipper sour Duke's relationships with the girls to get him to return to Hawaii in hopes of getting rescued. Riding a reverse tsunami back, unfortunately, Duke hits his head on a rock and loses his memories of The Castaways, forgetting them entirely and ruining their chances for rescue.
- Duke was played by actor Denny Scott Miller who also played Tongo the Ape Man in Our Vines Have Tender Apes.
- Duke Williams, Randolph Blake and Mary Ann Summers are all Kansas natives.