The manga is based on a dramatic tragedy diary written by Aya Kito, published shortly before her death. The diary, a true story based on her own life, was originally written in first person. It is about a girl coping with her teenage life along with a degenerative disease. She keeps a diary of not only what she does but how she feels and the hardships she must endure. Initially, the diary's purpose was for Kito to chronicle impressions she had about how the disease was affecting her daily life. As the disease progressed, however, the diary became Kito's outlet for describing the intense personal struggles she underwent in coping, adapting, and ultimately trying to survive her disease. As she notes in one entry, "I write because writing is evidence that I am still alive."

(Source: Wikipedia)