Seiyaku no Frontline

Summary

"Robot and boys' lonely Earth defensive battle..." Sai is a normal student, until one day a battle robot appears in front of him and tells him it is his mission to wipe out all humanity! If he does not accept or succeed, the robot would simply choose another person to do the job. But Sai quickly finds out that he can control the robot with his cell phone, and becomes determined to save humanity from its end!