That Child's Exchange Diary

Shim Min-Jin entered high school. Baek Ga-I and Geum Ho-Yang approached Min-Jin. They became friends with Min-Jin, who had been bullied in the past, and together they started writing an [exchange diary]. One day, while happily writing the diary, Ga-I was found dead. Just as Min-Jin was grieving, a new diary entry was uploaded. The author was... Baek Ga-I?

(Source: Naver, translated)