Killa Kan
Being a Grot is not a nice life. You are a scrawny, weak little being among big, aggressive Orks who love to kill stuff and who view anything smaller than themselves as inferiors. You are constantly yelled at and kicked, forced to do their drudge work, and your inevitable death is only good for a quick laugh. Throughout your life you cower in fear, hoping to avoid the wrath of your huge superiors, doing what they say to hopefully avoid being kicked, eaten or shot, all the while fighting with your fellow grots over scraps of half-eaten squigs. This often breeds spite and bitterness among many grots over the course of their often short lives.
But a precious few escape this fate. One day a Mekboy will scoop up some lucky grot, a Painboy will hammer some wires into the grot's brain, and he will be sealed into a 3-meter-tall, armed and armored water-heater on legs called a Killa Kan. Suddenly he's bigger, harder, stronger and shootier than all those boyz who abused him, and it's payback time. The only boyz he can't get after are those interred in Deff Dreds. Contrary to what you might think, the Mekboyz and Painboyz responsible for the Kan's creation view this initial revenge-spree of death and mayhem proudly, akin to a sort of validation of their efforts. Even aftter that, the Kan pilot still gets to take out his latent frustrations on anyone else the warband happens to be fighting, so everything's great for him. Of course Kan pilots are still every bit as cowardly as they were before, but that doesn't always matter when you think you're invincible.
Killa Kans are so favoured by Gretchin (I mean, who wouldn't love to control a hugeass war walker armed with all sorts of weaponry?) that whenever the resident Big Mek constructs a new one they often hold lotteries to find out who's going to get bolted in to pilot the thing. Only witnessing the actual wiring-in process can get most Gretchin to rethink their career decisions - but as mentioned above, by then it's usually too late. On the other hand, once the process is finished the Killa Kan is free to mutilate and kill his former tormentors, so that has to be worth something after all.
Questions about how the implanted grots feed themselves or defecate are probably best not contemplated.