Hydra's third sister forms one distinct head of the multi-headed monster created by Agastia's Monster Development Department. She possesses striking blue eyes and blue hair reaching chest length. Significant budget and time constraints forced major design modifications to Hydra, eliminating her planned venom-spitting capability and reducing the total heads from eight to four.

Among her siblings, the third sister exhibits a notably calm and rational personality, especially when sober, contrasting with their more aggressive and commanding natures. Her eyes, appearing deep pitch-black, visually underscore this trait of relative sanity amidst the group's frequent internal conflicts. However, this composure can become unstable under certain conditions, such as alcohol consumption.

Operating within Hydra's shared consciousness, she frequently engages in or mediates disputes between the other heads during missions and daily activities, reflecting compromises in her development. In combat, she demonstrates a clear preference for defensive strategies, prioritizing group cohesion and protection over direct confrontation. Her interactions extend beyond resolving internal strife, including participation in group activities like aquatic testing at the beach alongside other department monsters.