Tokyo 卍 Revengers: Baji Keisuke Kara no Tegami is a Manga/Manhwa/Manhua in english language, ACTION series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.cc fastest updating comic site. The Summary is

“Tokyo Revengers“ official spin-off! The story of Keisuke Baji and Chifuyu Matsuno, from their first encounter to their separation. Chifuyu Matsuno, the First Division vice-captain, has not yet accepted the death of Keisuke Baji, the First Division captain, who died in the battle against Valhalla. At that time, a letter from Baji before his death arrives at his hand.