Tokyo卍Revengers: Baji Keisuke Kara no Tegami

The official Tokyo Revengers Spin-off!!

A story about Chifuyu Matsuno and Keisuke Baji that wasn't revealed in the main story, from their first encounter to their final goodbye. Vice-captain Chifuyu Matsuno still can't accept the death of Keisuke Baji, captain of the 1st Division, in a conflict with Valhalla. That's when he receives a letter from Baji back when he was still alive.

(Source: K-Manga)