Well, no, the Valeyard isn't a companion, unless you count companion as someone who's traveled in the TARDIS and, um, to explain what he actually is would involve spoiling you for that arc. Which I probably shouldn't do. And the Doctor only wishes the Master was his companion ;) But I was including them as examples of non-Rose characters that influence huge gobs of plot. The companion-ness (companionicity?) of UNIT is arguable, but I choose to treat them as such even though only Jo ever gets to ride in the TARDIS because they do assist the Doctor (when he doesn't assist them) and the old name for companion was assistant.

And I forgot Turlough. *facepalms* Who actually was a companion. Sigh.