Their Shared Diary

Sim Min-jin enters high school and is approached by Baek Ga-i and Geum Ho-yang. For Min-jin, who was bullied in the past, the two become her first real friends and together they start writing a shared [exchange diary]. Then one day, while they're happily keeping up with their entries, Ga-i is found dead. Just as Min-jin is stricken with grief, a new diary entry is uploaded. The author is Baek Ga-i?