Robot soldier

Humans at the time are at war with an alien race. The aliens that humans refer to as "the grays" are far more advanced than humans. However, due to humanity's sheer will to survive, they've been able to hold their own against the aliens. During a battle on a distant planet that's far from Earth, you fought in a human army against the Grays during the battle you had been killed in action. You sacrificed yourself to ensure the safety of your squadron. This sacrifice allowed your allies to regroup with a larger platoon. Due to your efforts, courage, and sacrifice, the captain of the Flagship that's orbiting the planet deemed you a worthy candidate of becoming a mechanised soldier. (I know it seems pretty copy and paste. It's made by an amateur but I hope it's at least enjoyable.)