That's the mystery of him, we don't know. However he was friends with the Master and was 8 when he looked into the Time Vortex and 90 when he visited the Medusa Cascade. The best information from TV is that he was a prankster among pranksters; the star outcast in a fraternity-like organization which was known for producing outcasts.
in the key to time serial it's stated that he graduated with a 51% on the 2nd try.
It's also strongly implied that, before she turned nasty, he had something going with the Rani.