Aseria serves as the primary spirit warrior for the kingdom of Rakios in the parallel world of Phantasmagoria. Recognized as the strongest spirit in service to Rakios, she wields a massive Eternity Sword named 'Existence' in combat. Her personality blends stoicism and taciturnity, though punctuated by moments of naivety and ditzy behavior.

Upon the protagonist Yuuto Takamine's arrival in Phantasmagoria, Aseria immediately engages him in combat during their first encounter. She later saves his life in an early battle against enemy spirits, establishing her dual role as a formidable warrior and protector. Throughout subsequent conflicts, she fights alongside Yuuto under the command of Princess Restina Dai Rakios.

Existing as a tool for human warfare without personal autonomy, a condition she accepts without overt protest, Aseria embodies Phantasmagoria's societal structure where spirits face human discrimination despite their essential battle role. She demonstrates unwavering loyalty to Rakios' leadership and fulfills her duties without questioning the human manipulation of spirits.

During a critical enemy assault on Rakios castle, Aseria participates in the defense but proves unable to prevent significant losses, including the king's death and the kidnapping of Yuuto's sister. When Yuuto falls into despair afterward, she uses a headbutt to physically snap him out of his emotional state, demonstrating her unconventional approach to support. Her interactions with fellow spirits, like the energetic Oruphaliru and nurturing Esperia, highlight her place within the spirit cohort despite her reserved nature.

Her background beyond her service to Rakios remains unexplored in the available media, including the two-episode OVA adaptation. Her development centers on combat engagements and functional relationships within Yuuto's team rather than individual history or evolution.