"Kokoro Library" is a slice-of-life manga series set in a remote, tranquil library perched on a hilltop. 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 been adopted into the family as a baby. Her arrival is shrouded in mystery, as she was found with a single book and a note bearing her name. The library itself is a place of quiet reflection and connection, where the sisters foster a sense of community and warmth.

The narrative unfolds through episodic chapters, each highlighting the sisters' daily lives and interactions with visitors. Kokoro, despite her cheerful and earnest demeanor, often grapples with feelings of inadequacy and a desire to prove her worth to her older sisters. Iina, the eldest, is responsible and pragmatic, often taking on a maternal role, while Aruto, the middle sister, is more carefree and artistic, providing a balance to the trio. Together, they navigate the challenges of running the library, from mundane tasks like organizing books to more personal moments of growth and self-discovery.

Visitors to the library include a diverse cast of characters, each bringing their own stories and struggles. These interactions often serve as catalysts for the sisters' reflections on their own lives and relationships. Themes of family, belonging, and the transformative power of books are woven throughout the narrative, emphasizing the library as a space where people find solace and connection.

The series maintains a gentle, contemplative tone, with a focus on character development and emotional resonance. The sisters' bond is central to the story, with their individual personalities and dynamics driving much of the narrative. While the plot is largely episodic, there is an underlying thread of Kokoro's journey to understand her origins and her place within the family. The library itself becomes a metaphor for the heart, a place where stories and emotions are shared, and where the characters—and readers—find moments of quiet introspection.