Eruna Ichinomiya is the main protagonist of her series and a first-year student at Mikagura Academy. Her decision to enroll at this particular school was largely impulsive, driven by her hobby of playing gal games during junior high school. After her cousin Shigure Ninomiya showed her a pamphlet for the academy, Eruna became instantly captivated by the photograph of Seisa Mikagura, the model used in the material, and decided on the spot that she was destined to attend.

Personality-wise, Eruna is defined by her incredibly high tension, overwhelming optimism, and a tendency to overreact to nearly every situation. The word lose does not seem to exist in her vocabulary, and she consistently throws herself into challenges with boundless energy and a belief that she will succeed. She is cheerful and extroverted, possessing a magnetic personality that allows her to easily befriend even the most aloof or intimidating individuals, such as Seisa Mikagura or Kyoma Kuzuryuu. While she often appears to be an airhead, Eruna can be surprisingly perceptive and serious when it comes to understanding the feelings of others, and she never hesitates to lend a hand to someone in need.

The most defining aspect of Eruna's character is her intense and often comedic attraction to cute girls. This interest is a direct influence of the gal games she used to play, and it is the primary lens through which she views many of her female classmates. She frequently fantasizes about romantic relationships with girls like Seisa or Otone Fujishiro, and is prone to physically glomping those she finds particularly adorable, such as Himi Yasaka. In addition to her affinity for girls, Eruna also displays fujoshi tendencies, often becoming excited and reading romantic subtext into the interactions between male characters like Yuto and Asuhi or Usamaru and Tonkyun. She is also known to be a big eater who claims she does not easily gain weight, and she enjoys being spoiled and praised, especially by the cute girls she admires.

Eruna's role in the story is set in motion when she is invited by Seisa to join the Going-Home Club on a trial basis. She is thrust into the academy's unique battle system, where club representatives fight using special powers to determine their club's privileges. Despite not being skilled at complex battle strategies and tending to rely more on her physical abilities, Eruna proves to be a formidable and unpredictable fighter. Her primary power, known as Toy Gun, allows her to shoot flash bullets from her fingers. She later develops this ability into Toy Bayonet, which summons a bayonet made of light and grants her incredible speed. Official statistics rate her growth expectation and unpredictability at maximum levels, highlighting her potential.

Throughout the series, Eruna forges key relationships that define her journey. Her primary motivation is her idolization of Seisa Mikagura, whom she often calls Goddess, and Seisa takes a special interest in Eruna's development. She shares a close, teasing friendship with Yuto Akama and Asuhi Imizu, and she forms a significant bond with the club-less Otone Fujishiro, becoming her first real friend at the academy. Her relationship with her cousin Shigure is one-sided, as he dotes on her constantly while she is often embarrassed or disgusted by his overbearing affection, though she admits he knows her better than anyone. She also develops a close bond with Bimii, the cat-like teacher who, despite being the frequent victim of her pranks, acts as her supporter and advisor. Over the course of the narrative, Eruna demonstrates significant development, culminating in the fifth novel where she creates her own club called the After School Paradise Club and becomes its representative.