Muv-Luv Unlimited

Shirogane Takeru finds himself stuck in a parallel world where mankind is on the brink of extinction after battling against alien invaders known as the BETA for the last 20 years.

Takeru soon finds himself enrolled into the United Nations 11th Force to fight these aliens. In this strange world he finds parallel versions of all of his high school friends, with one exception: Kagami Sumika, who does not seem to exist.

Soon he will have to fight to save mankind and protect the girl he comes to love over the course of the story.