Actually, we do sort of know.
That Prisoner was going to originally be a recruitable character (Kato said so in one of his interviews), but he was cut because the game couldn't really handle yet another backgroundless character with a weird accent (you know he would have been given an accent if he were a PC). Thus, we can ask ourselves, "Given that they include a cave-woman who isn't Ayla, would they then turn around and include a cave-man who was Kino"?
Additionally, given that Dragonopolis was from the future, it is unlikely that he would have still been alive.