Serika Ginyūin is a teenage idol who initially visits Hatagaya Academy for its school festival before transferring as a student and residing in the Quest Dormitory. She possesses thigh-length black hair typically tied in a right side ponytail with a large pink and yellow ribbon, square bangs framing her forehead, and lilac-purple eyes. To conceal her identity upon entering the academy, she wore sunglasses; her appearance is frequently described as beautiful, contributing to her idol popularity.

Her core aspiration is becoming a singing idol, a dream ignited six months prior when, feeling despondent on a swing before a performance, she encountered a mysterious young man in a face-obscuring purple hoodie. He offered her a tissue and declared she possessed a "Success Flag," inspiring her renewed vigor and optimism. She calls him the "Purple Flagman," crediting him for her subsequent successful performance, and deeply desires to thank him.

At Hatagaya Academy, Serika sought Souta Hatate’s help evading pursuing fans but developed a negative perception of him, mistakenly believing his numerous female friendships indicated womanizing. This led her to treat him with rudeness and callousness, starkly contrasting her typically sweet, kind, and levelheaded demeanor evident in her easy friendship with Nanami Knight Bladefield. She sensed vague familiarity around Souta but couldn’t pinpoint its source.

A turning point came during Christmas at the Quest Dorm. Reluctantly spending time with Souta and Nanami for perceived protection, Serika voiced her criticisms until Souta’s familiar smile prompted her to search his belongings. Discovering the distinctive purple hoodie, she confronted him; his nervous reaction confirmed he was the Purple Flagman.

This revelation shocked Serika, who felt intense guilt. She profusely apologized for her prior harshness, reversing her attitude to address him respectfully as "Hatate-sama" with admiration. Expressing deep remorse, she suggested self-punishment and broke down crying over insulting her benefactor. After Souta and Nanami departed, she picked up the discarded hoodie, smelled it, and quietly called Souta a "jerk" for withholding his identity, blushing at the unrecognized proximity.

Serika then developed overt romantic feelings for Souta, mirroring other girls’ affections. She blushed heavily in his presence, finally understanding his kind and helpful nature. She actively sought interactions, indicating a desire to ask him an important question. Observing his kindness to the group solidified her feelings, leading her to join festive greetings.