[Manga] Bokurano (Complete)

Summary: During a summer campus, 15 children, 8 males and 7 females, find a grotto by the sea; inside there's a computer and some electronic equipment, and soon they meet the weird owner, Kokopelli, who claims to be a programmer who's working on a brand new videogame, in which an impossibly large robot has to defend the Earth against 15 alien invasions, and proposes that the children test it.

Only one of the children doesn't sign the contract, as her brother doesn't let her; after they've all signed the contract, they find themselves on the shore, and think it was a dream.

But soon a giant robot appears, and a small creature, who claims to be their guide, teleports them inside their robot, where they assist to a fight between the weird man and the robot, and soon after the man disappears.
They'll slowly learn the mysterious rules of the game.