Iku Kasahara is the main character of the series. She is twenty-two-years-old and is initially ranked Librarian First Class, but is later promoted to Chief Librarian. She joined the Library Defense Force's Kantō Library Base in 2019 after being inspired by a high ranking Kantō Library Defense Force member who saved a book she wanted to buy that was targeted for censorship. She holds this unknown person in high regard, thinking of him as her "prince", and she wants to be an "ally of justice" as he was for her. As such, she has a strong sense of justice when it comes to freedom of expression and is willing to put her life on the line for the books she has sworn to protect. When she enters into recruit training, however, she finds it to be very challenging, made especially so by her drill instructor Atsushi Dojo who pushes her harder than most other recruits. In high school, she had been a member of the track and field team, so she has a lot of stamina and drive to continue with the Library Defense Force.

When the story begins, Kasahara is a sub-par recruit who constantly makes fatal mistakes and is not as knowledgeable about the cause she is in the middle of compared to others around her, mostly due to her not paying attention in lectures on the base. Despite these apparent flaws, she is recruited into the base's Library Task Force, an elite group of soldiers who go through rigorous training in order to respond during difficult operations. While at first she starts out slow, she soon becomes capable of clerical tasks in regard to working in the base's library, though still finds it difficult to make a strong presence on Dojo, her superior officer.

After quite some time of working under Dojo and receiving great care from him (which she didn't expect), her feelings began to soften and she began opening up to Dojo.

Later on thanks to Satoshi Tezuka, Kasahara found out that Dojo is actually her prince. At first, she was so confused and got extremely awkward around him, but thanks to Komaki, she realized that she had to accept her prince for who he really is. She also admitted to herself that even if Dojo is not her prince, she might've fallen in love with him.
In the movie, they both confess and kiss twice. At the end of the movie, it is implied she and Dojo got married.