Sui Usami is a student at Shiranui Private High School, a special institution where demi-humans study to learn how to become fully human. She is a rabbit demi-human, distinguished by her long, fluffy ears and a small, rounded tail. Her appearance includes two-tone hair, and she typically wears the school's uniform, which features wide sleeves and bright white tights. Her movements are often described as graceful, and she has a mischievous expression that can be both provocative and charming, sometimes sticking out her tongue in a playful challenge.

Sui is known for her sharp tongue and tendency to speak her mind without a filter. She frequently uses blunt or harsh language, telling others to shut their trap or calling things disgusting. Despite this prickly exterior, her actions often reveal a more caring and motivated individual. She is not cruel for the sake of being so; rather, her defensiveness seems to be a way of maintaining distance. She has a clear sense of right and wrong, is protective of those she considers friends, and does not hesitate to confront someone she believes is acting inappropriately, such as when she scolds her teacher for scaring another student.

Her primary motivation for wanting to become human is deeply rooted in a personal debt of gratitude. She is driven by a desire to repay the kindness of a human who helped her in the past. This goal is so significant that it forms the core of her character. When the person she holds dear becomes critically ill, her desperation to see them one last time forces her to confront the limitations of her existence as a demi-human, as she is physically unable to leave the school grounds.

In the story, Sui serves as one of the central students under the charge of the misanthropic teacher, Rei Hitoma. While she initially clashes with him and dismisses his attitude, she ultimately must place her trust in him during her moment of crisis, revealing her hidden secret and begging for his help to see her dying benefactor. Her key relationships include her fellow classmates, such as Tobari Haneda, a shrike girl with a sunny disposition, and her teacher, whose own journey away from misanthropy is intertwined with the needs of students like Sui.

Sui Usami does not possess supernatural combat abilities but her identity as a demi-human likely comes with enhanced physical traits associated with rabbits, such as agility and powerful hearing, though these are not explicitly detailed as offensive powers. Her notable ability is her resilience and strong will, which she demonstrates by pushing past school rules and her own pride to save the person who once saved her. Her development revolves around moving from a guarded, sarcastic student to one who learns to rely on her human teacher, creating a mutual path toward healing for both of them.