Iku Kasahara is the central protagonist of her story, a young woman whose life was profoundly changed by a single, heroic act she witnessed as a high school student. During her senior year, she was at a bookstore when agents of the Media Betterment Committee, a censorship enforcement body, attempted to seize a book she planned to buy. A member of the Library Defense Force intervened, retrieving the book and inspiring her with his courage and dedication. She has since held this unknown savior, whom she calls her prince, in the highest regard, and his example became the driving force behind her decision to join the Library Forces herself.

Born in Mito City in Ibaraki Prefecture, Iku grew up as the only daughter with three older brothers, an upbringing that contributed significantly to her outgoing, competitive, and sometimes rough-around-the-edges personality. This background, combined with her participation in track and field teams throughout high school and university, gave her exceptional athleticism, stamina, and drive. Standing at 170 centimeters tall, she has a model-like physique but typically dresses very casually and is unconcerned with traditionally feminine presentation. Her passionate and impulsive nature often leads her to act before thinking, a trait that has earned her nicknames such as "hot-blooded idiot" and "mountain monkey." Her fearless and aggressive response to a training exercise involving a simulated bear attack gave her the moniker "Kill Bear Kasahara, while another incident where she continued to smile after a confrontation left her splattered with blood resulted in the name "Bloody Kasahara.

Iku is defined by an unwavering, almost fierce sense of justice regarding the freedom of expression and the protection of books. She is willing to risk her life for the texts she has sworn to defend and will occasionally disobey direct orders if she believes her superiors are making a morally wrong decision. Despite her tough and clumsy exterior, she possesses a kind and loyal heart, a duality that is particularly evident in her interactions with her commanding officer and eventual love interest, Atsushi Dojo. Although she initially finds him to be a harsh and critical instructor who constantly pushes her harder than others, she gradually develops deep feelings for him, eventually discovering that he is, in fact, the very prince she has been searching for all along.

At the start of her career, Iku is assigned to the Kanto Library Base with the rank of Library Clerk First Class. She is initially portrayed as a sub-par recruit who struggles with academic lectures and makes frequent mistakes. However, her raw potential and determination are recognized, and she is recruited into the elite Library Task Force, a special unit that undergoes rigorous training to handle dangerous operations. There she works directly under Dojo, who serves as her superior officer. Her development throughout the story sees her grow from a clumsy novice into a capable and respected member of the task force, eventually earning a promotion to Library Clerk Supervisor.

One of the key conflicts in Iku's life involves her family, particularly her mother, who strongly disapproves of her dangerous and unladylike career choice. This opposition stems from an incident in which Iku was seriously injured as a child, and it has created a significant rift between her and her parents, leading to a long period of estrangement. She has a close and supportive friendship with her roommate, Asako Shibasaki, a library counter staff member who often provides a more grounded perspective on Iku's romantic and professional troubles. Iku shares a strong bond with her fellow task force members, including Satoshi Tezuka and Mikihisa Komaki, forming a tight-knit team that functions as a surrogate family.