If you want to find out who the Doctor really is then lock him in a room with his greatest enemy: the Daleks.
This has been one of the few truths of Doctor Who as a show, even more so after its return in 2005 and the introduction of the Time War plot establishing that the Doctor sacrificed his own people to rid the universe of the Daleks once and for all. As an unyielding trickster, the Doctor thrills in subverting the universe’s expectations and inspiring others to do the same. This trait is embedded deeply in the Doctor’s fictional history. He’s not going to bow to the constraints of Time Lord society, he’s going to steal a TARDIS and get the hell out of there. He’s not [whatever his Gallifreyan birthname is], he’s the Doctor. Just the Doctor. And you can’t force him do anything he doesn’t want to do.
Unless you’re the Daleks.