Karuta Roromiya is a significant character in Inu X Boku Secret Service, known for her unusual demeanor and deep loyalty. She is a descendant of a Gashadokuro, a giant skeleton yokai formed from the bones of people who starved to death, which directly influences her supernatural abilities and her defining trait of perpetual hunger. Her physical appearance in her human form is that of a slim teenage girl with salmon-pink hair typically styled into pigtails and matching maroon-colored eyes. She is most often seen in the standard dark uniform of the Secret Service, which includes a black coat and pants with a gray undershirt and red tie. In a later reincarnation as a twenty-year-old, her hair is cut shorter to neck-length, and her figure has matured slightly, though she still favors a similar blazer style.

On the surface, Karuta presents as an exceptionally quiet and emotionless individual who is almost always seen with some form of snack or food in her mouth. This detached and zoned-out appearance often leads others to underestimate her, but she is, in fact, very clever, perceptive, and keenly aware of her surroundings. Her fixation on eating is a central part of her personality; she enjoys food almost to the point of obsession and can consume vast quantities without apparent consequence, a trait linked to her Gashadokuro heritage. However, her concern for others runs deep. She has shown great care for her friends, like Ririchiyo Shirakiin and Banri Watanuki, and will act decisively to assist them when they are in need. Though she rarely displays strong emotions, she is capable of crying during moments of extreme relief, grief, or happiness, demonstrating a hidden depth of feeling beneath her placid exterior.

Karuta's primary role in the story is as a Secret Service agent. Initially, she is assigned to protect Kagerou Shoukiin, a master she serves with instinctive loyalty, seemingly unbothered by his sadistic nature and his habit of treating her like a pet as long as she is provided with food. Her most revealing relationships are with two other characters. Her bond with Banri Watanuki is particularly significant; they have been friends since childhood, and their relationship is described as more than friends but less than lovers. Watanuki is deeply in love with her and has trained to become stronger specifically to protect her, a sentiment she quietly reciprocates through subtle gestures of care and intimacy that often leave him flustered. She also forms a genuine friendship with the initially isolated Ririchiyo Shirakiin, which begins simply when Ririchiyo shares her lunch with a hungry Karuta. This kindness forges a loyal bond, with Karuta showing sincere concern for Ririchiyo’s well-being, a feeling that is warmly returned.

Throughout the narrative, Karuta undergoes notable development. As a child, she disliked her menacing yokoi form, especially in contrast to Watanuki's cute transformation, and after an accidental transformation caused a quarrel with her friends, she decided to hide her true feelings away to prevent future incidents. In the first part of the story, she remains largely reserved. After a traumatic event where she is possessed by an enemy youkai and asks Watanuki to kill her to stop herself, the story shifts to a reincarnation arc. In this new life, her personality is depicted as more cheerful and expressive, though she retains her characteristic detachment and love for food, now acting as a playful caretaker to a younger Watanuki.

Karuta's notable abilities are derived from her supernatural lineage. She can transform into her true form as a Gashadokuro, becoming a gigantic skeleton wreathed in a purple aura. In this state, she possesses immense size and strength, as well as the ability to teleport or move at imperceptible speeds, making her a formidable agent. Despite this terrifying power, her human talents include a sharp, intuitive mind and an exceptional skill for cooking, where she prefers to adapt to her ingredients rather than follow rigid recipes.