Echidna reigns as the Demon Queen, leading forces from the ruined Demon World. Her initial form possesses long pink hair, golden eyes, black horns, and attire featuring a red dress, thigh-high boots, and a black belt with a golden suspender. After defeat by Leo Demonheart, her physical form shifts to a younger girl, though she retains her original mature appearance in specific contexts.

Her leadership mandates compassion and restraint, explicitly forbidding unnecessary violence against humans during invasions and minimizing casualties in conquered territories. Despite her army's antagonistic role, she holds no inherent hatred toward humanity and welcomes humans into her ranks. Her primary objective is obtaining the Philosopher's Stone to restore habitability and peace to the polluted Demon World, driven by childhood stories of a demon advocating coexistence.

She ascended to rulership reluctantly, motivated by tales of a legendary hero and a peace-seeking demon. Formative experiences fostered her commitment to creating a demon realm free from conflict, underpinning her invasion of the human world as a necessary evil to secure the Stone for her people's survival.

Her relationship with Leo Demonheart evolves significantly. Initially, she despises him for annihilating her army, reacting violently to his presence or name. Yet, she acknowledges his defensive motives and gradually recognizes their shared goals after he infiltrates her forces disguised as the Black Knight Onyx. During private interactions, including a dinner where he questions her motives, she reveals her vision for demonkind and aversion to bloodshed, unaware of Onyx's true identity.

A critical juncture arises when Leo's bioweapon programming threatens global destruction. He reveals his heart is a Philosopher's Stone and implores Echidna to kill him to save both worlds. Despite the opportunity to seize the Stone and fulfill her mission, she refuses, unable to sacrifice someone sharing her aspirations for peace. This decision cements her acceptance of him into her army and signifies a shift from vengeance to mutual guardianship.

Her bonds with the Four Generals—Shutina, Lily, Mernes, and Edvard—highlight her empathetic leadership. She deeply values their loyalty, privately supports their struggles, and organizes personal gestures like dinners to express gratitude. She mediates conflicts, such as Mernes's initial vengeful intentions toward humanity, reinforcing her role as a unifying figure.

Echidna wields formidable magical abilities, including high-level fire spells like Inferno, metamorphosis for disguise, and the mana-intensive Hero-Binding Spell designed to neutralize Leo. Her combat prowess is tempered by a strategic preference for minimizing harm.

Her character development culminates in rejecting cyclical violence. By sparing Leo and embracing coexistence, she redefines her quest for the Demon World's prosperity, aligning with her core principles of kindness and sustainable peace.