Help! My Pilot's a Psycho!

The narrator wakes up as a mecha. Not a pilot in a mecha, but the machine itself, a metal shell with no body inside it and a mind that stays awake through every sortie. Monsters have gotten into the world, and through several campaigns the narrator quietly pushes its own battle intent through whoever is at the controls, steadying aim and pressing an attack the pilot never planned. That should have earned some kind of standing. Instead the core takes damage in a fight with a monster, performance falls off fast, and the unit gets written down as scrap and handed to the training yard for rookies to practice in. The narrator settles for seeing out its remaining service quietly. Then the latest trainee arrives, a teenage girl with a violent streak who runs drills at a pace the frame was never rated for. She has no interest in what the machine thinks about any of it, and the gun barrels keep overheating.