Seira Kokubyakuin reigns as Seikou Academy's student council president and dominates the school's popularity rankings, holding the top spot for three consecutive semesters among all students. Her striking appearance features fair skin, silver-grey eyes, and ankle-length silver-grey hair adorned with two small purple flowers, typically complemented by the standard female school uniform.

She maintains a polite, proper demeanor consistent with a refined lady, addressing others with the respectful honorific "-san" and using phrases like "Oh my." A perpetual smile masks her true emotions. Though outwardly helpful, she exhibits a mischievous streak by manipulating situations involving Kanade Amakusa's "Absolute Choice" curse—alternately offering subtle hints to assist him and deliberately worsening his predicaments, such as publicly declaring romantic interest to damage his reputation.

Seira possesses specific knowledge about Kanade's curse beyond others' awareness, though her motives for aiding or hindering him remain ambiguous. She recognizes non-human entities, identifying Chocolat's true nature upon their first encounter and advising her to act on her feelings for Kanade. Her dialogue references divine connections, directly mentioning the "God World" and gods, implying supernatural knowledge or influence. She tasked Himeru Hisoka with seducing Kanade as part of an unspecified plan, accepting the failure without surprise and hinting at backup strategies.

Her name's etymology reflects her character: "Seira" blends "clear, pure" (清) with "lightweight fabric, gauze" (羅), while "Kokubyakuin" merges "black and white/right and wrong" (黒白) and "hospital" (院).