It's amazing how few people notice his trousers (that are related to the fact that his role as savant includes the function of jester). Although he is a mutant, that's not related to his outward appearance. He's just a hunchbacked cripple, much like thousands of Imperial citizens who've suffered from birth defects that are well within the limits of purity (and, in fact, the seal on his chest is there to testify to that fact).
His mutation is much more subtle than that and even his Inquisition masters aren't completely certain of its extent. Some suspect that he is more than three thousand years old, but records are inconclusive and it may be that his name and hereditary defects are simply passed on along from father to son. Either way, it is his extraordinary memory and unbreakable loyalty that make him valuable to the point that inquisitors have fought and died to control him.
Yes, I do have a lot of other models to share eventually. But I'm way too busy at the moment (Summer holidays, commissions to paint, house redecorating to complete) to take the photos I want to. So you'll all have to wait until my new studio is finished (yep, part of the redecorating means that I get my own studio!) and I have a permanent photo-box set up.