The Unraveling of Doctor Man's Identity and Past in Bioman
Doctor Man was probably not all that enigmatic in Bioman. Maybe some people figured out he was a human being since day one (he is) though at that time, I assumed that the Big Three and Farrah Cat were human beings themselves. But let's focus on the unraveling of Doctor Man's identity of who he was as a human.
In Bioman episode 19, the title of the series was "My Father is Doctor Man" which was really a huge hint Doctor Man was a human being. In that episode, he introduced Prince not as his creation but as his beloved son. When he first appeared in that episode, Mason considered Prince's claims to be nonsense before he got that hilariously funny incident with Mettlzer with both returning to the Neograd Fortress in really bad shape, he was shocked to see Prince there. Doctor Man called Prince his only son which made Mason say in his thoughts, "Wait, I didn't know Doctor Man was a human being." which made me think, why in the world does he think Dr. Man is a robot?
This of course leads to something that maybe Mason was programmed to know genetics. During the time Prince was preparing Grotesquekans and was surprised to see the plastic tube that he was made to use earlier, he later remarked, "Amazing. Being able to build a robot at his age. He is truly the son of Doctor Man." During that time, the Biomen had already assumed Doctor Man came from Japan. During that time also in the second half, Ryuta guessed it right that Doctor Man's wife wouldn't want her son to be heartless, which of course lead to Prince's being revealed as a robot replica of Doctor Man's real son who would soon appear. But at this point, Mason had already learned Doctor Man was a human being but not his full origins.
In Bioman episodes 25-26 we see Doctor Man's real son Shuichi who is using the assumed name Koichi. I do find the whole beginning a bit funny considering the Biomen would think a Mecha Human would sweat or even EATING NOODLES?! Enough of the comedy, well Doctor Man's robot son Prince when he was hit with Mettzler's rapier (which Mason tossed at him), he seemed human and bled. So Doctor Man's super science is hard to explain so just don't attempt to understand it since this is all a show. What we learn in there is that Doctor Man was a mad scientist who used some kind of experiment that turned him old really fast and he became a cyborg. At that point, Mason also saw his creator's origins and probably said to himself, "That's it! This is the last straw now we have proof he is a human! We can't tolerate this much longer." which leads to episode 28 where Doctor Man really had them all fooled into thinking that he died when he used a decoy of him to fool them. Despite the fact Doctor Man wiped their minds out, however it seemed they have accepted the fact he is a human being as later, Mason still remembered Shuichi is Doctor Man's son.
More of Doctor Man's past was unraveled as the series was coming to its glorious end. That is when we learn of Shiro Gou's missing father had returned and two, the fact he used to be a colleague of Doctor Man. This plot also had Shuichi working with Shiro's father Professor Shibata whose identity was revealed only in the finale arc. Doctor Man's past was revealed that he was more for power, Professor Shibata was more for peace. Hmmm I just love how this "little experiment" was later copied into Megaranger's Dr. Hinelar and Dr. Kubota.