Our hero is a teen struggling with various moral dilemmas about right and wrong when using his super powers. He has the ability to generate various mechanical stuff from his limbs using the nanites in his body. Actually the whole world is shown to be infected due to an incident but only he has the ability to command and control them. He can also cure others by diffusing their nanites. He is often seen comparing orders with moral consciousness about consequences. I love it all. And to add to it our hero is intelligent enough to take good decisions. He has Agent Six as a father figure to look up to, who looks like Agent Smith from Matrix but never the less, a bad ass mentor the kind not to mess with. He manages to balance Rex's silliness well. The side kick Chimp Bobo is also a likable character and adds to the mix. Doctor Holiday is a motherly figure although too pretty for Rex to accept, so he hits on her regularly. The wittiness is very good especially when it involves Van Kleiss.
I like stories which detour the least from main story line but with Rex even the detours are fun to watch. Overall one of the best action cartoons out there.