Do you remember? 🙂 At the end of their very first adventure, the Ninth Doctor (as Christopher Eccleston) offered Rose Tyler (as Billie Piper) an impossible choice: to join him.
I thinks the answer is… rather obvious, even without watching the video. 😀
LaterÂ Mickey Smith (as Noel Clarke) also joined as the Doctor’s companion, even it was for a short time.