Sono Uruwashiki Hito wa,

Koto, a very ordinary girl, is good friends with Fumi, the top student at an exclusive all-girls' school. Fumi is the perfect student, with her combination of talent and beauty, but after summer vacation, she suddenly disappears without a trace. Amidst the rumors circulating around the school, Koto discovers a diary. Hoping to solve the mystery of her friend's disappearance, what will Koto learn from the diary…?

(Source: Hakusensha, translated)

Note: Chapter count includes a volume-bonus chapter.