Steven made his first Dr Who appearence in "The Chase," at the end of which he snuck aboard the TARDIS and became a stowaway. In the next story, "The Time Meddler," he popped out of hiding and was eventually accepted as a crew member. He left the crew in "The Savages" to become a mediator between the two civilizations that lived on the planet...well, they didn't say what planet it was.
Steven was played by Peter Purvis. Born Feb 10, 1939. After leaving Dr Who, Peter became a "Presenter" (which in the USA would be an MC or Moderator) first for a popular children's show called Blue Peter and then for sports shows, and was highly successful.