Hydra, one of Valhalla's eight princesses from the Dolphin Constellation's royal family, clashes as a rival to Valkyrie with her brash, short-tempered, and tomboyish demeanor—opposing Valkyrie's gentleness. Her pursuit of Valkyrie to Earth stems from Valkyrie rejecting their father's political marriage; Hydra fears being forced to replace her. Crashing into Akina Nanamura's shrine prompts Akina to seal her powers, shrinking Hydra into a child-sized body while retaining her adult consciousness and memories. Akina alone can temporarily break this seal, reverting Hydra to her adult form for inconsistent durations.

In adulthood, Hydra wields flight, superhuman strength, energy manipulation, and devastating energy blasts, alongside transforming into a pink-and-yellow winged horse. Prolonged Earth residency softens her antagonism: she masters kendo as a skilled swordsman, befriends locals, and cohabits with Akina. Their dynamic shifts toward cooperation yet remains contentious, often erupting in physical fights when Hydra mocks Akina's breast size. Extreme arguments fuse them into the combined warrior Akidra, merging their powers and personalities—a state reversible if they harmonize. Hydra maintains a platonic bond with Kazuto Tokino and casual ties with his sister Rika.

Hydra's resentment toward Valkyrie over the marriage crisis evolves into genuine friendship, rooted in shared sword-training at a Valhallan academy. Other princesses like Laine and Chorus irritate her without deep enmity, while Valkyrie's maid Sanada garners wary respect. Ultimately, Hydra integrates into Earth life, defaulting to her child form unless Akina lifts the seal for specific needs, completing her journey from headstrong antagonist to a tempered, albeit fiery, ally.