Yes, we have another stowaway. Adric is a native of E-Space, and in "Full Circle", while escaping from some bad guys, he hides aboard the TARDIS. He's the fourth stowaway after Steven, Zoe, and Sarah Jane. Adric remains a TARDIS crew member when Romana and K9-II stay in E-Space, though the doctor's regeneration into Peter Davidson, and then up to the "EarthShock" story when he is killed while trying to save the earth from collision with a giant Cyberman freighter.
Adric is reminiscient of Zoe. They were both mathematical geniuses, but they were still both kids, which meant that they did a lot of zooming around and getting into trouble.
Adric is played by Matthew Waterhouse. He was born on December 19, 1961. He was one of the youngest companions and was allegedly the first long-time, card-carrying Doctor Who fan to actually become an actor on the show. Clearly a sign of how long Dr Who had been on the air even in 1980 when Matthew joined the cast. He was also (allegedly) the first "openly gay" actor on the show.
After Dr Who, Matthew has been a successful stage actor and writer. I'm beginning to think that a tour of duty on Dr Who is good for your career since so many of the companions did well after they left.