Hydra's Sisters are the multiple heads attached to Hydra, a monster created by Agastia's Monster Development Department. Originally designed with eight venom-spitting heads featuring regenerative capabilities, budget and time constraints led to a final form with only four additional heads mounted on a humanoid female body.

Each head possesses a distinct personality and consciousness, resulting in frequent internal arguments and bickering. This constant friction complicates coordination and makes coexistence difficult for the shared body. Their inability to agree or cooperate is a defining characteristic.

Hydra, including her heads, was intended as a combat monster to oppose heroes like Blader. However, the design compromises—reducing the head count and removing venom-spitting—significantly reduced her combat potential. The heads' persistent infighting further impacts operational cohesion during deployments.

Their depiction remains consistent as a collective entity defined by internal strife stemming from their separate personalities within the shared form.