Otto is a walrus serving as a crew member on Captain Silver's pirate ship, the Pork Saute. He participates in pirate activities, including the capture of Jim and his companions. His character develops unexpectedly when he encounters Baboo, the infant son of Admiral Bembo's innkeeper, who stowed away on Jim's boat. Otto forms a strong bond with the baby, voluntarily embracing the role of Baboo's guardian and protector during the journey to Treasure Island.

This protective bond triggers a shift in Otto's loyalty. During the climax on Treasure Island, as Captain Silver moves to betray his crew and the protagonists to seize the treasure, Otto recognizes Silver's duplicity. He turns against his former comrades, confronting and holding off pirate crewmates to defend Jim and his friends. His intervention contributes to the pirates' surrender once they understand Silver plans to abandon them without the treasure.

Following Silver's defeat and the discovery of Captain Flint's treasure within the sunken ship revealed by the drained volcanic lake, Otto joins Jim, Kathy, and the reformed pirates. He helps recover the treasure and departs the island aboard Silver's ship with Jim, Kathy, Gran, and Baboo, leaving Silver and his monkey helmsman, Spider, stranded on makeshift log boats.

Otto's background before joining Silver's crew is not explored, and no further appearances or developments beyond the film's events are documented. His character arc is defined by the transition from pirate crew member to compassionate guardian and ally, motivated by his attachment to Baboo and his rejection of Silver's treachery.