Bokurano Ours Vol. 1,2,11(Manga) (3 Books) 
VIZ MEDIA LLC
(W/A/CA) Mohiro Kitoh
By Mohiro Kitoh
Synopsis:
One summer, fifteen kids innocently wander into a nearby seaside cave. There they meet a strange man who invites them to play an exciting new video game. This game, he explains, pits one lone giant robot against a horde of alien invaders. To play the game, all they have to do is sign a simple contract. The game stops being fun when the kids find out the true purpose of their pact.