The Ancient Magus' Bride: Midsummer Pilgrimage

A non-human being lives alone in London at night.
One night, in the fog, he meets a girl asking for help. She has lost her memory and doesn't know why she is there, so he saves her from the darkness and the two become friends.

Wanting to help the first friend he has ever made, the non-human Sol heads to the homes of "Elias the Thorn Wizard" and "Chise," whom he has heard rumors about from fairies, in order to restore the girl's memories.