Evadne manifests as a demon sword, the "Demon Sword of Black Flames," capable of assuming human form. In this human guise, she takes the shape of an adult woman with hip-length black hair and eyes shifting between purple and grey. She consistently displays a morose and taciturn demeanor.

Shifting into her true flamberge form requires the specific chant: "Shake off your slumber, cloak yourself in darkness, bring yourself the end and kill God." This transformative ability marks her as one of the rare demon swords possessing such a trait, a fact noted by others of her kind.

Her origins connect her to the Empire. She was held by Charlotte E. Firobisher, an illegitimate daughter of the Emperor, though suggestions point to a possible origin from Siegfried. Evadne primarily serves as Charlotte's silent companion.

Within the narrative, she represents a significant figure for Aria, the demon sword of wind, due to their shared nature as transformable demon swords. Aria seeks answers from Evadne regarding their existence and origins.