Erstin Ho is a student at Garderobe Academy, where she holds the rank of seventeen in the Coral class, though she is later demoted to rank thirty. She hails from the country of Annan, and her family is wealthy enough to afford her tuition at the prestigious academy. Physically, she is a teenage girl with green eyes and neck-length blonde hair, often styled with distinctive cheek curls. She is a quiet and reserved individual, with a timid nature that is complemented by a sweet and friendly disposition. Despite her reserved personality, she has a strong desire to connect with others, which draws her to more lively and active people like her roommate, Arika Yumemiya.

In the story, Erstin serves as a roommate to both Arika and Nina Wang. She initially struggles with hesitation in making friends due to personal reasons, but she eventually forms a strong bond with both girls, becoming a close friend to each. Her role within this trio is often that of a bridge, mediating between the clumsy and spontaneous Arika and the introverted, obstinate Nina, who do not get along very well. She is acutely aware that her own skills as an Otome are not as remarkable as those of Arika or Nina, so she chooses to support them in the best ways she can, offering comfort and encouragement. She quickly befriends Arika and helps her adjust, and she also provides emotional support to Arika when she is troubled by her feelings for Sergay. Her supportive nature extends to risking her own safety, as seen when she shoves Arika out of the way to take a shot of acid meant for her, suffering only minor injuries herself, though she strangely chooses not to report the perpetrator.

Erstin harbors a significant hidden romantic affection for Nina. This feeling is demonstrated most clearly during a pool scene when a creature swims around the girls; believing Nina is behind her, Erstin indicates that she is comfortable with the contact because it is her. Her feelings are further implied when she performs CPR on Nina in a particularly affectionate manner after Nina nearly drowns.

A major motivation for Erstin is her desire to protect the friendship she has built with Arika and Nina. She keeps a dark secret that threatens to shatter this bond, as she is revealed to be a member of the Schwarz organization. In this capacity, she is able to summon a creature known as a Slave. During a critical battle, she decides to use her Slave not to fight, but to intervene and prevent her two friends from clashing with each other. Ironically, this desperate act to save their friendship proves fatal. The Slave she summons is killed by Nina, and because of the magical link they share, the destruction of the Slave also results in the death of Erstin herself. Her family has a long-standing devotion to the god of knowledge and technology, which ties them to the larger conflict against Garderobe. One of her key relationships is with the antagonistic Tomoe Marguerite; Erstin discovers that Tomoe is responsible for a series of strange accidents targeting Arika, and her silence on the matter adds to the tragic pressures she faces.