Charlotte Pollard

Charlotte Pollard, the Edwardian Adventuress, is probably the most popular of the Big-Finish-created companions. She's had runs with two separate Doctors, and as of this writing, is still popping up again for more fill-in-stories. However, for a little while, she had a series all of her own, mostly dealing with the Nick Briggs creations, the Viyrans. The first series did well enough that a second came out three years later. Maybe I'll listen to them eventually.