ＧＡＯＧＡＯ！１ｓｔ ~ラジカルシークエンス~

The protagonist of the game is a college student named Yuuji Shibata. One evening, just after hearing on the news that someone has stolen a tiger, he goes out to a convenience store. On the way back, in a dark alley, he notices a strange creature, a young girl with cat ears and tail. She doesn't seem to remember anything about herself, except her name beginning with the syllable "Mi...", which afterwards earns her the nickname Mi. Yuuji brings her to his house and tries to introduce her to the world of humans. (which, naturally, also involves techniques of human procreation). But will Yuuji be able to keep Mi as his companion (sexual and otherwise), or will he have to face her mysterious past?..

[From Moby Games]