Hydra, one of Valhalla's eight princesses, begins as a rival to Princess Valkyrie. She pursues Valkyrie to Earth to avoid replacing her in an arranged marriage. After crash-landing on Akina Nanamura's shrine and damaging it, Akina seals Hydra's powers as punishment, shrinking her to child size. Unlike Valkyrie, whose mind regresses in this form, Hydra retains her sharp adult intellect and personality.

Tomboyish, headstrong, and short-tempered, Hydra acts impulsively, sparking constant clashes with Akina. To repay the shrine debt, she lives alongside Akina, frequently teasing her about breast size—a habit met with physical retaliation. Despite their friction, they forge a close friendship. Adapting to Earth, Hydra masters kendo, showcasing her combat prowess.

A unique dynamic emerges: intense arguments fuse Hydra and Akina into Akidra, a formidable warrior with merged abilities and separate internal consciousnesses. The fusion dissolves if they become overly agreeable. Akina can temporarily lift Hydra's seal to restore her adult form, though its duration wavers. Once, this transformation embarrassed Hydra when her child-sized clothing strained against her adult body.

In the "Bride Training Boot Camp" OVA (SPECIAL release), Hydra engages in simulated marital scenarios, partnering with Akina to prepare a wedding ceremony. Her relationships include a competitive yet evolving bond with Valkyrie, annoyance toward Princesses Laine and Chorus, wary respect for chief maid Sanada, and friendships with Kazuto Tokino and his sister Rika. Yet her central connection remains Akina.

Throughout her journey, Hydra shifts from a solitary rival hunting Valkyrie to an integrated team member, joining beach trips and crisis responses. She embraces collaboration, notably against threats like Valkyrie Ghost, who nearly kills her in one encounter.