Arnold (Children of the Evil Eye)

Arnold was a boy from Earth who worked as a jailer during Oswald's regime.

Biography
When the Third Doctor was taken to prison prior to being executed, he met Arnold and convinced him to help him escape, but they were caught. When Oswald was overthrown, Arnold was released and the Doctor invited him to join him in the TARDIS. (TVC: Children of the Evil Eye)

After they dropped off Oswald on another planet, the Doctor and Arnold tried to help a fleet of ships that appeared to be heading directly into a supernova. This turned out to be an illusion. The fleet was crewed by the Spidrons. After escaping the Spidrons' city, the Doctor and Arnold encountered the primitive, ape-like natives who also lived on the Spidron world, and worked with them to attack and destroy the Spidrons. (TVC: Nova)

After this adventure, the Doctor returned Arnold home. (TVC: The Amateur)

Behind the scenes

 * In the text, Arnold was at first said to be from the 32nd century, then later to be from the 31st.