Jemmy the Cat serves as the central antagonist in the 1975 Japanese anime adaptation of Hans Christian Andersen's story. A cunning, malevolent feline, she despises Marina, the mermaid protagonist, and ceaselessly schemes to destroy her. Though her motivations remain opaque, fan theories propose jealousy or a belief that Marina threatens her bond with the prince, whom she accompanies as a favored pet. Jemmy first confronts Marina with aggressive hissing upon the mermaid’s arrival on land, later escalating to sabotage her interactions with the prince. She deliberately startles Marina’s horse during a forest ride, causing a fall that exposes her to a wolf attack—a crisis thwarted by the prince’s intervention. Undeterred, Jemmy allies with the royal chamberlain to expose Marina’s presence, manipulating the king and queen into opposing their son’s affections. Her final scheme pressures the monarchy to address the prince’s defiance, after which she vanishes without resolution. The film leaves her fate, origins, and post-narrative existence undefined, confining her role to a persistent yet ultimately ineffective adversary. Speculation about ties to the sea witch arises from shared English dub voice actors, though narrative inconsistencies and separate Japanese casting contradict this theory. As an original antagonist absent from Andersen’s tale, Jemmy distinguishes the adaptation through her unique, villainous presence.