5. Jenny
Jenny. But not the Moffat era, high-kicking, Silurian-loving Jenny Flint. This Jenny is the Doctors Daughter. (Well, created from a sample of his DNA.) Her only appearance came in series 4's 'The Doctor's Daughter' where she met her Father, showed enthusiasm and bravery to help the people of Messaline and then flew away in an escape shuttle never to be seen again. Where she went and what she did next is a loose end begging to be picked up in the future but, for now, the intrigue and possibility of another Time Lord being out there with a thirst for adventure and discovery makes her one of the shows most interesting one-off characters.