|Origin||The Grim Adventures of Billy & Mandy|
Grim's Diary is an important object that appeared in Grim Tales
Grim kept a diary in his time when he was with Billy and Mandy. He wrote down his adventures with them and how he felt wih them, especially with Mandy. Grim Jr. eventually found it in one of his vaults and started to read in it.
The book also is the family's family album.