The man's original personality, a man named Prot, was lost forever. K-PAX still referred to himself as Prot occasionally, but only when talking to Humans. Prot's history was relatively unremarkable, he lived in a house on the edge of a town in America with his wife and daughter. He worked in a nearby food processing plant but returned home one day to find his house had been broken into.
His wife and daughter were dead, and the criminal was still inside the house when Prot entered. The man attempted to bat Prot with a crowbar but his attack was swatted aside and Prot floored him. Within seconds, Prot was able to snap the man's neck in revenge, but the act of murder, along with the loss of his family, drove the man insane.
When he returned to the city, he was a changed individual. After attempting to commit suicide it became apparent that he had undergone an immense psychological change. At first psycologists thought that he had created the persona of an alien from 'K-PAX' to escape the pain and anguish he suffered, but it soon became apparent that this was not the case.
Prot, now calling himself K-PAX, believed himself to be from the planet K-PAX and that he had arrived on Earth to study the planet. He claimed to have been to Earth several times and was able to describe in detail each body he had used. Records linked these to similar cases across the country, and before long Prot was gone. He even took one of the residents of the menal home in which he was staying with him to K-PAX.
The League has never been able to find a planet with intelligent life that refer to themselves as K-PAXians. It is therefore theorised that, if K-PAX does exist, it exists outside the League's Seven-Galaxy sphere of influence.