Kokoro is the youngest of three sisters living in and running a remote mountain library. At ten years old, she has just started her role as a librarian and takes pride in sharing her name with the institution.

She has a positive, hardworking, and innocent nature. Her main interests are reading books—especially those by the author Himemiya Kirin—and tending to flowers. She can create rainbows using a watering can. Her personal room is decorated with many cat motifs.

She likes to eat French toast, pudding, and cake, but dislikes celery and cauliflower. Her birthday is on March 18.

Both of her parents are deceased. Her mother died shortly after her birth, and her father passed away when she was very young. Her father was a soldier who stayed in the area after a war to help with rebuilding; he later founded the library. He named it "Kokoro," meaning "heart," after his wife, a nurse he met during the war. The names of the three sisters—Iina, Aruto, and Kokoro—were chosen to form the phrase "kokoro aru to ii na," meaning "it's good to have a heart."

She is learning the work of a librarian, which includes shelving books, helping visitors find materials, and organizing promotions to attract more people to the library. Her personal journey includes finding her father’s wartime diary, which helps her connect with her family’s past and the library’s original purpose. This discovery becomes important in trying to stop the town mayor from closing the library.

She lives with her older sisters, Iina and Aruto, and has a cat named Kit. Her friends include Akaha Okajima, a regular visitor to the library, and Hikari Inoue, another young trainee librarian.