Shinigami Tsuki no Shinpu-sama wa, Hiroware Musume Nado Aisanai

Victor, an immortal, lives in a secluded, abandoned church. Having lived in solitude for 90 years for a certain purpose, he takes up the responsibility of raising Maria, an abandoned child with nowhere else to go. Burdened by the curse of never loving a human, Victor is constantly whispered to by the Grim Reaper who lurks behind him, "Never love a human."
https://mechacomic.jp/books/273121