Linebarrels of Iron

Kōichi Hayase, a weakling and an average student, is always bullied by his stronger classmates, and ends up being protected by his childhood friends Risako and Yajima. This makes him feel even worse since he realizes he is weaker than Risako, the woman he likes. But all this suddenly changes when a falling orbital drop mecha called Linebarrel misses the landing zone and kills him. He then returns to life, thanks to the intervention of the alien woman Emi, who was trying to land the mecha. Kōichi ends up being the pilot of Linebarrel and is linked to it for the rest of his new life. Linebarrel was designed to save humanity from an approaching interdimensional war and to bring justice to Earth—but with a pilot who has been resentful all his life this isn't top priority.

Synopsis from Anime News Network