Himitsu Kichi de Himegoto

Having become a university student, the protagonist moved to a tranquil town.

When he got used to living in a new environment, he happened to find a girl
going deep into a mountain. Following after her out of curiosity
he found something that looks like a building.

"I-is anyone here?"

This was the first encounter with Yuki-chan for the protagonist in her secret fort.
Since then he has played with her using toys. One of these days, he finds
a porn book hidden in her fort. It seems like Yuki-chan is interested in H things...

"I'll show you my d*ck if you were interested in those things."

The two's secret activities begin here...

From Dlsite English