Gogo no Midori de Machiawase

When Chikage was a child, his best friend, Ray, disappeared without a trace—along with everyone else's memories of him. Now a high school student, Chikage happens to run into Ray again and is shocked to find that his childhood friend no longer looks human. In fact, Ray was never a human in the first place, but a guardian deity of the mountain and forest. Scared to lose him a second time, Chikage decides to keep meeting him in secret...

(Source: Shinshokan, translated)