Board Thread:The Reference Desk/@comment-188432-20130310185635/@comment-188432-20130725175315

Why? Just because they share one story, they're not necessarily companions. They can be, if, as with Ly-Chee, the one story is implied to have contained multiple adventures. But if it's just one adventure in one story, I'm not sure that quite qualifies. Witness that Jayne Kadett is not considered a companion of the Tenth Doctor.