Duke the Whale functions as an enforcer directly serving King Triton within the underwater kingdom. His responsibilities include monitoring Princess Marina's actions, especially her interactions with the human world, and reporting his observations to the king. He maintains order and enforces Triton's rules concerning surface interactions. Duke appears specifically when Marina returns from the surface after meeting Prince Eric, informing King Triton of her actions, thereby influencing narrative developments tied to her choices. Despite his title, Duke is explicitly identified as a whale, not a shark. His role remains consistent throughout his appearances, with no significant evolution or expanded backstory provided. No further appearances, development, or background are documented in other media beyond the original film. He is voiced by Kosei Tomita in Japanese and Neil Shee in English.