Iku Kasahara is the main character of Toshokan Sensō; 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.