Isekiden: Legend of the Ruins

Once every few decades, the gate to a lost city with untold treasure is opened for a limited time. Adventurer Ruka has come to the ruins of this city to see what he can see. But there is a condition: the gate can only be traversed by a young girl. His quest seemingly over before it begins, Ruka is helped by a mysterious fox spirit. She lends him the power of a girl... by turning him into one himself!