This is probably the story of Atsushi being adopted by Hashira of the Demon Slayer Corps when he was a child, and later being stripped of all his money by the Vampire Technique and moved to Yokohama, where he met Dazai by chance.
Including worldview: Demon Slayer, Fumino, Qishen, Curse, Conan.
Background setting: The background of Demon Slayer has been changed from Taisho to Heisei. Atsushi is set in the same year as Mizuho, and the timeline is mainly set by Bunhao.
Cp: tooton
Purpose: World peace