ヴァージン・トリガー

One day a girl appears from the window. She asks main character to fight together to protect the Earth. She starts to train together with protagonist until the day of decisive battle.

Aliens fight a proxy war for control of planets by creating "triggers" who must do battle with each other. They are androids with human hearts. Haruka is a trigger girl that the protagonist must train to be ready to defend the earth. The final battle will be determined based on how you have trained and treated her.