Hydra's fourth sister functions as one of the multiple heads comprising the Hydra monster entity. Originally designed with eight venom-spitting snake heads, the Hydra was reduced to four heads—including the fourth sister—due to budget and time constraints. These heads share a single body yet harbor distinct personalities, sparking frequent internal bickering.

Voiced by Satsumi Matsuda, the fourth sister shares vocal casting with Hydra's second head, implying potential similarities in vocal traits or personality, though no explicit distinctions between them are documented. Collectively, the heads exhibit aggressive and commanding tendencies, contrasting with the head possessing pitch-black eyes, which displays relative calmness when sober.

The fourth sister’s specific role, unique personality attributes, and individual background remain undefined across all media. No narrative arcs, standalone actions, or story developments focus exclusively on her beyond her existence as an integrated component of Hydra’s composite form.