Ace was a companion of the seventh Doctor. She was quite possibly the greatest companion ever (in my not so humble opinion). She carried explosives. She once took out a Dalek with a baseball bat. She was the companion when the original show was canceled, and the last episode had her and the Doctor heading out together for more adventures. In the quasi-canonical tie-in novels which were published for quite awhile after the original show went off the air, it's very strongly hinted that Ace eventually graduated into being a sort of honorary Time Lord herself, and went off on her own adventures.