Jemmy, a talking cat, serves as a major antagonist in the narrative. He is the pet of a prince but exhibits cruel, jealous, and mean tendencies, directing a particularly strong hatred toward the mermaid princess Marina from their first encounter on a beach. His actions are consistently driven by this animosity. He attempts to eliminate Marina on multiple occasions, once by jumping onto her horse during a forest ride, causing the animal to throw its rider and leave her surrounded by a pack of wolves. The prince intervenes and rescues her, thwarting the plan. Later, Jemmy is observed perched on the shoulder of the Lord Chamberlain. Together, they discover the prince with Marina, and Jemmy declares that she will not escape again. He goes directly to the king and queen to report that the prince is rejecting an arranged marriage and is in love with another girl, advising that the situation must be resolved to prevent it from becoming more serious. This action contributes directly to Marina’s arrest for treason. Jemmy’s ultimate fate is not shown, as he does not appear again after these events and is apparently still alive. The motivation for his intense hatred is not explicitly defined. Some speculation suggests he may have viewed Marina as a threat to the prince, though simple personal dislike is also considered a possibility. There has been debate among some fans that Jemmy could be the Sea Witch in disguise, a theory partly fueled by both characters sharing an English-language voice actress. However, this seems unlikely as the Sea Witch is not portrayed as evil in the film, and the original Japanese version uses different actresses for each role.