Manga by author Ayamine Maito

Olivia is picked up by a death god, Z, deep in the forest as a child and trained by him in combat for reasons unknown. One day, Z disappears, leaving behind a letter instructing Olivia to head to the Kingdom along with a sword bearing the qualities of a death god's weapon. Thus, the girl lacking in common sense, knowledge, and normalcy sets off to join the army.

.Shinigami ni Sodaterareta Shoujo wa Shikkoku no Tsurugi wo Mune ni Idaku manga summary: Olivia is picked up by a death god, Z, deep in the forest as a child and trained by him in combat for reasons unknown. One day, Z disappears, leaving behind a letter instructing Olivia to head to the Kingdom along with a sword bearing the qualities of a death god's weapon. Thus, the girl lacking in common sense, knowledge, and normalcy sets off to join the army.