Eiko is a first-year student attending Babyls Demon School, where she belongs to Class A. Physically, she is a demon girl of average height with long, slightly frizzy blonde hair that features uneven bangs, large yellow eyes, pointed ears, and black bull-like horns. She wears the standard female uniform of the school, which includes a pink capulet and a red bow fastened with a distinctive cat-eye clasp.

Initially a quiet background figure, Eiko gains narrative prominence following a chaotic public duel between Iruma Suzuki and Alice Asmodeus. During this commotion, Iruma rescues her from harm, an act that immediately inspires in her a deep sense of gratitude and a fervent, unspoken affection for him. Despite her strong feelings, she is plagued by persistent misfortune and bad timing, which prevents any direct contact with Iruma for a significant period. Later, she finds herself in another dangerous situation and is saved by the student council president, Ameri Azazel. This second rescue reshapes her feelings as well, sparking a new admiration and a romantic interest in Ameri, leading Eiko to eventually acknowledge her feelings for both Iruma and Ameri.

Eiko's personality is defined by a contrast between her sociable nature with her peers and her extreme shyness around the individuals she idolizes. She is known for her romantic daydreaming, often becoming lost in thought about her crushes to the point of being oblivious to their actual proximity. Her determination is a core trait; despite her awkwardness, she persistently attempts to connect with Irumu and Ameri, though these efforts are frequently played for comedic effect due to her unfortunate luck. This same resilience, however, serves her well in more serious contexts. In the high-stakes environment of the Harvest Festival, a survival-based exam, she relies on her sharp observational skills and stealth to successfully endure the competition, rather than raw power.

Her role within the story evolves from that of an unseen admirer to a more active supporting figure. While she lacks the central importance of the main trio, her presence contributes to the world-building and comedic texture of Babyls. One of her most significant relationships forms with Asmodeus Alice. Their acquaintance begins with initial friction, primarily over her attempts to photograph Iruma, but this conflict dissolves when they realize they share a mutual and profound worship of Iruma. This common bond transforms them into close friends and collaborators who frequently communicate and work together to compile images and information about their shared hero. She also maintains a close friendship with a classmate named Gaako, who acts as a patient anchor for Eiko's more excitable tendencies.

Over the course of the narrative, Eiko demonstrates notable development. Academically, she proves herself to be highly competent. Her skill at creating efficient study guides and organizing information not only helps her friends but also allows her to achieve outstanding results on school exams, ranking tenth in her year. This success leads to a practical increase in her status, allowing her to rise from the initial rank of Aleph (1) to at least Bet (2) on the student ranking system. Her abilities are specialized rather than combative. She excels in silent observation, information gathering, and meticulous organization. Her hobby of organizing her hair accessories by color reflects her personality, with a notable inclination to use the blue tones associated with Iruma and the red associated with Ameri. Her dedication to documenting her interests is so intense that it is paralleled in an alternate universe spin-off, where she takes on the role of an undercover journalist. Information about her family mentions a younger brother named Eita, though the nature of their relationship is not explored.