Akuma de Tenshi: Umare Kawatte mo Soba ni Itai

A serious angel named Ell is sent to retrieve a former angel who became a devil named Allen. After choosing death to escape his dull existence in hell, Allen awakens with a baby angel in his arms. Ell is there to guide Allen through a ritual that could purify his soul and return him to heaven. However, Allen shows no interest in returning. One day, he recalls the angel Allen from his past, and despite having no memories and now being a devil, he still feels a connection. At the moment Ell's heart races with emotion, he suddenly transforms into an adult.