Vasilisa, a high-ranking magician of the Russian Orthodox Church, serves as mediator for Annihilatus, the church's organization dedicated to hunting forbidden spirits and supernatural entities. Her name originates from the Russian fairy tale "Vasilisa the Beautiful," where the heroine gains aid from a cannibal witch. Bishop Nikolai Tolstoy accuses Vasilisa of being that very girl who obtained an ageless body through occult means; she deflects this by insisting she is merely a "fairy tale heroine" and "national idol."

She appears as a tall woman with blonde hair and white skin "just beginning to show signs of decline," typically wearing a conservative red nun's habit and white gloves. Though resembling someone in her late twenties, her true age is unknown, and her reactions to age-related questions suggest great antiquity. Her personality combines childish enthusiasm for Japanese otaku culture—manifested by forcing her subordinate, Sasha Kreutzev, into cosplay outfits like those from *Magical Powered Kanamin*, provoking Sasha's violent reactions which leave her unfazed—with ruthlessness in combat and unwavering loyalty to her charges.

As Annihilatus's mediator, she claims her position stems from being its strongest magician, overseeing operations and personnel like Sasha, whom she assigned her signature revealing outfit. Her magic draws deeply on Russian folklore. Primary spells invoke the "Old cannibal woman of the one-legged house" from her namesake tale, unleashing effects like the "Skull Lamp," a ring of explosive flames that expands violently to incinerate targets, and "Witch Shadow," summoning a monstrous entity resembling the cannibal witch to attack foes. She also wields the "Water of Life," a legendary substance granting regeneration, youth, and longevity. This enabled her survival when Bishop Nikolai attacked her with the "Water of Death," regenerating lost body parts effortlessly.

Vasilisa first appeared during the Academy City Invasion Arc, monitoring Sasha after the latter became a vessel for the angel Gabriel's residual Telesma. Her fixation on Japanese culture surfaced when she attempted to gift Sasha a *Magical Powered Kanamin* costume during a critical briefing. Later, at the Anglican Church–Russian Orthodox Church Prewar Conference, she served as the Russian Church's formal representative, while Sasha handled informal diplomacy. Her loyalty was tested during the British Royal Family Arc when the church ordered Sasha's capture to appease Fiamma of the Right. Vasilisa rebelled, tearing up her church contracts, declaring she would betray the institution to stand by Sasha. She aided Sasha's escape and single-handedly fought Skogsfru, an Annihilatus operative sent to apprehend them, asserting her dominance as the organization's strongest combatant.

In the World War III Arc, Vasilisa confronted Bishop Nikolai Tolstoy in Moscow. Nikolai attacked her with the Water of Death, dissolving half her body, but she regenerated using the Water of Life and killed him for plotting against the Patriarch. After rescuing the imprisoned Patriarch, she coordinated with the fallen pope Matthai Reese to counter the Star of Bethlehem threat, casually remarking on the Patriarch's cuteness.

During the Magic God Othinus Arc, she and Sasha guarded Patriarch Krans R. Tsarskiy at a United Nations conference in New York. While bodyguarding, she discussed the Gremlin threat with Kaori Kanzaki, expressing skepticism about informants like the Dawn-Colored Sunlight while acknowledging shared interests in opposing Gremlin. Upon learning of Gremlin's invasion of Tokyo, she bypassed the chain of command, ordering her subordinates to crush Gremlin operatives in Russia and protect civilians. This move angered Sasha, who attacked her. Vasilisa endured the assault unharmed and reaffirmed her commands, emphasizing her intent to shield the Patriarch from morally compromising decisions.