Princess of Cathedral

The mechanical civilization has been destroyed and the world is infested with immortal monsters called "Monotsuki". Isana, a young girl born in a remote village, wanders into an ancient ruin where her sister has disappeared and encounters an android that has no memories. Convinced that her sister is still alive, Isana gives the android the name "Hime" and together they set off on a journey to find her sister.

(Source: Ichijinsha, translated)