"Zetsuko does not know exactly what happened, when he got to the survivor scientist's lab he found very little besides pieces of broken bodies and...things and, and some fire, Zetsuko likes fire...it's so warm, poking around Zetsuko did find a bit of notes on the experiment aside from the soul, it mentioned something they referred to as [i]Hyde's serum[/i], probably named after Dr. Jekyll who made a serum to suppress his evil but in doing so became [b]Hyde[/b], his evil, younger, alter ego, as he took this serum Hyde became more powerful and it did have an effect on his appearance, making him taller...The serum is likely named after Hyde as it increased strength and had it's own affect on a subject's appearance, as seen in Rory"

"...so this is Zetsuko's theory: The survivors were implanting the soul not as a means to change him into a survivor, but instead as a way to control him, they attempted to implant it along with giving him a serum to make him stronger than the other HUSKs, that way even with the limited number they can capture they could still beat the Oasis Forces, that serum is what would have made him stronger and is likely what affected his appearance. after that is when Rory had gone insane and killed the survivor scientists and destroyed the lab in the process."

"as for his whereabouts, Zetsuko has two ideas: either he was found by the Oasis forces and they allowed him into patrols after recognizing his enhancements and that they had no access to the serum for themselves,overlooking his artificial soul as it is likely connected to the enhancements, as Rory destroyed it along with the lab; or, Rory is still wandering aimlessly with yet to be found by either the SURVIVORS or the Oasis Forces."

[@BeanieBaggie]
(so it's basically like a performance enhancer type thing, you know like steroids, but different...)