Ikunyuushu ~Genjuuya Advance Episode~

Main character is a young man who lives in a mansion in the suburbs of the Highlandy Republic capital. The mansion used to be an enterprise to provide girl escorts from aquatic tribes, and protagonist starts to study aquatic culture. One day a hunter appears and asks protagonist to take care of a mermaid. That was the first encounter with Ryuka.

[From vndbreview]