Mermaid no Kisetsu

The game follows Masato Takahata, a high-school sophomore, living alone, going to school classes and working a part-time at Aqua Rouge cafe. The setting is in the near future with technological advancements, but the living style didn't change that much. One day you find a strange note with an address of a home page. You check it out only to see the ominous warning text. While you're not trying to figure out the mystery behind it, you will traverse town meeting various people, mostly girls and women, with whom you'll work on relationship to become friends with, or something more.

[from MobyGames]