Kokoro is the youngest of three orphaned sisters who live in and operate the remote mountain library that is their family home. She has just become an official librarian alongside her older sisters Aruto and Iina. Kokoro was orphaned very young; her father died in a war when she was a small child and her mother passed away shortly after her birth. She is depicted as a young girl with very long purple hair and gray eyes. Her name, combined with those of her sisters, forms the phrase kokoro aru to ii na, meaning it is good to have a heart, a motto chosen by her parents. Kokoro possesses a gentle, kind, and optimistic personality. She is deeply polite, wise beyond her years, and goes out of her way to help visitors to the library. She is passionate about literature and firmly believes in the power of books to comfort and inspire people. Her primary motivation is to honor her parents by keeping the library running and understanding what it truly means to be a librarian. As the protagonist, her role revolves around learning about the world, her family, and the library's legacy while facing the everyday challenges of her duties. A significant part of her development comes from the conflict surrounding the library's potential closure. When a mysterious locked book belonging to her father is stolen and the town mayor announces the library will be shut down due to lack of visitors, Kokoro is heartbroken. The book, which contains her father's war diary, is eventually returned. Reading it, she learns her father was a soldier named Sant Jordi who helped rebuild the town after the war and constructed the Kokoro Library. This discovery gives her a profound personal connection to her parents and becomes the catalyst for saving the library. She shares a close and supportive relationship with her sisters Aruto and Iina, working alongside them to maintain their home. Kokoro also possesses a unique, gentle ability to produce rainbows with her watering can, a small miracle that symbolizes the warmth she brings to her surroundings. Through her journey, she grows more confident in her role, learns the strength of community, and ultimately inspires the townspeople to rally against the library's closure, allowing the three sisters to continue their peaceful life together in the Kokoro Library.