Asuka Utsunomiya, a 17-year-old student at Gokurakuin Girls' High School, stands 145 cm tall and weighs 30 kg. She possesses lilac or purple hair, brown eyes, and a distinctive mole, typically seen in her school uniform of a vest and bow. Her demeanor is aloof and reserved, rarely showing expressions or smiles, and she speaks in a matter-of-fact tone, exuding a mysterious aura.

Deeply immersed in spiritual practices and mysticism, Asuka engages in fortune-telling, exorcism, and discussions of supernatural phenomena. This expertise leads others to seek her assistance with occult matters. A significant facet of her character is her romantic and sexual interest in Aya Akabane, though she seldom acknowledges it directly. She frequently leverages her mystical pursuits as a pretext for initiating physical interactions with Aya. Examples include licking Aya's face to expel an "evil spirit" causing hiccups, performing intimate acts disguised as unconventional treatments like a "cold cure" involving kissing when Aya had the flu, and kissing Aya's cheek during an evening encounter.

Her actions unintentionally disrupted Aya's plans during a pool incident, leading to Aya nearly drowning; this occurred in Asuka's presence, though she wasn't directly involved in the rescue. In the OVA set during a hot springs trip, she made advances toward Ai Nanasato while others slept, consistent with her pattern of initiating physical contact under ambiguous or opportunistic circumstances. Her interactions consistently blend her esoteric interests with underlying romantic motivations, particularly seeking closeness with Aya.