In Nip/Duck, Kat's X-rays are shown revealing to viewers what Kat really is. This was the proof that Coop needed to stop Kat, but in the end it was Kat who won yet again. This had been the closest episode to getting rid of Kat so far.
Kat's body is part android and part "organic but weird". We can only hypothesize the functions of the numerous machines within Kat: the two motherboards stuck to his brains might be enhancements for his intelligence or what he needs to control the robot part of his body (or maybe both), the two machines inside his belly could be a bionic heart and a robotic stomach and the device in his throat might be the source of his corrosive saliva. His bones appear normal, although we do not know if he has a skull. He also appears to have no bones in his tail and this could be the reason he is able to move it so well.
Most people think the three organs in his chest are three lungs, but some have speculated that they could be three hearts. Though this is unlikely: during the vet's examination in Nip/Duck he has no heartbeat (he has to concentrate really hard to generate a 'normal' heatrate for the vet to listen to).
It is unknown whether his paws and his eyes are mechanical or not, but they quite obviously are according to what Kat does in several episodes.
Coop wonders if Kat is an alien or a cyborg, but it's pretty obvious that he's both. He might have been turned into a cyborg by the other aliens to become more powerful for his mission, or maybe all the inhabitants of his planet are bionic like him.