"Kokoro Library" is a 12-episode anime series set in a remote, tranquil library perched on a hill surrounded by nature. The story revolves around three sisters—Iina, Aruto, and Kokoro—who manage the library and assist visitors in finding books. The youngest sister, Kokoro, is the central focus, having recently joined her older sisters after growing up in an orphanage. Her arrival introduces a dynamic of adjustment and bonding as she learns the routines of the library and the responsibilities of being a librarian.

The narrative unfolds in a slice-of-life format, with each episode presenting self-contained stories that explore the sisters' daily lives, their interactions with visitors, and the occasional challenges they face. The library serves as a hub for various characters, including regular patrons and travelers, each bringing unique requests or problems that the sisters help resolve through their knowledge of books and their empathetic approach. Themes of family, community, and the quiet joys of rural life are woven throughout the series.

Iina, the eldest sister, is responsible and nurturing, often taking on a maternal role. Aruto, the middle sister, is more carefree and mischievous, providing comic relief and a contrast to Iina's seriousness. Kokoro, the youngest, is earnest and eager to prove herself, though her inexperience sometimes leads to mistakes. Her growth as a librarian and her deepening bond with her sisters form the emotional core of the series.

The tone is gentle and reflective, emphasizing the soothing atmosphere of the library and the surrounding countryside. While the plot lacks high-stakes drama, it excels in portraying the quiet, meaningful moments of everyday life. The series also subtly explores the power of books to connect people, solve problems, and provide comfort, reinforcing the library's role as a sanctuary for both the characters and the audience.