3. Torchwood's Top Agent

Captain Jack Harkness was introduced during the 9th Doctor's run as a former Time Agent that became a time-travelling scam artist, who was inspired by the Doctor's heroism and decided to fight on the side of the angels. He was a companion for a time, until becoming immortal and later teamed up with the 10th Doctor. He had a spin-off in the adult-themed Torchwood series, but his roots are in the Whoniverse. Bring him back for a few episodes, since the Torchwood series is in limbo. His presence always felt less like a companion and more like a team-up, since he was such a heroic figure.