Bandit

Bandit was discovered by (protagonist here) while venturing through one of many ruined cities. Having been on his own even since before the destruction, he knows what it's like to do whatever it takes to survive. He wasn't always black and white, but somewhere in the mix of destruction and death going on during the rampage for the region he lived in, his physical appearance was altered a little. He still can use the same moves, same battle tactics, and so on - the only difference was instead of being orange and red is he is now black and white. 

It had been a long while since Bandit had seen something other than a Pokemon. Even then, the surviving Pokemon were far and few between, and many that were left had undergone even more changes than he had - some fusing into completely new Pokemon, or at least combining the looks and attributes of two or more Pokemon. Most of these new hybrids he tried to steer clear of, as he noticed they were fairly unstable and were prone to sudden, violent outbursts. 

On top of (protagonist here) not being strictly a Pokemon, they were able to understand him completely. He wasn't sure if that was because of some new fangled technology, because of what they were and what they looked like, or if it had anything to do with almost everything else in the world being changed to some degree. Regardless, they seemed nice enough and proved to be trustworthy, so at least for the time being, Bandit decided to unofficially tag along.