Men at Work!

The game takes place in a Middle-Age western European town. At the daytime it is a very active and joyous town. However at night it's another story. After sun sets, monsters will lurk all around town which raises concern about the safety of the townspeople. As a result the town has set up a guild of hunters who go around at night and kill these monsters. Most of these hunters are doing so for money, and they rarely reveal their true identity to anyone. The player takes on the role of Wil, who in daytime is a geeky part-time history teacher, but at night is a strong hunter.

[From Anime Densetsu]