Eris is a sixteen-year-old scout sent by the Catian people to serve as an advance observer on Earth. As a member of the Catian race, also known as Catia, she possesses feline features including a tail and cat-like ears, which she can use alongside her technology to prove her extraterrestrial origin to those who doubt her. Her physical appearance is notable for her tall stature and fair skin, complemented by red eyes and light orange hair. While adjusting to the customs of her new environment, she maintains a friendly and cheerful demeanor, easily befriending the humans she meets as well as the domestic cats of Earth. This pleasant and approachable nature is balanced by a strategic mind and a deliberate cultivation of harmlessness that masks her true competence.

Eris is motivated primarily by her mission to learn about Earth and establish friendly relations, as well as her personal interest in the human boy Kio Kakazu. The kindness he shows her leads her to select him as a potential mate, a decision influenced by her impending first heat cycle. This biological imperative becomes a significant source of internal and external conflict. When her first heat begins to disrupt her diplomatic duties as an ambassador for her people, she is required to take medication to prematurely end it, with the understanding that she will have another opportunity during her second cycle. This situation highlights the tension between her personal desires and her professional responsibilities.

In the story, Eris serves as the catalyst for the central events, being the alien whose arrival draws the attention of various factions, including government agencies and rival alien races. Despite her sometimes naive or airheaded demeanor, she is a capable and competent fighter who employs advanced Catian technology and unfamiliar combat strategies when the situation demands it. Her character develops through her deepening bond with Kio, which solidifies into genuine romantic love after she learns that he risked his life to rescue her when she was captured. This leads to a key moment where, in response to another character expressing jealousy over her closeness to Kio, she reveals that Catian law places no restriction on the number of lovers, indicating her acceptance of a polyamorous arrangement rather than bearing ill will toward her rivals. Her notable abilities include her proficiency with Catian weaponry and technology, her combat skills, and her role as a skilled anthropologist capable of observing and learning from human culture despite her outwardly playful personality.