1. The Other Doctors
Seriously, how many characters in Doctor Who are of a medical profession? There's a comparison to the Doctor with all of them. Martha's journey from drooling over to Ten to saving the world single-handedly reflected the line in her first episode, when she remarked she'd be a doctor too one day. Similarly, Rory the nurse was always second best to the Doctor, until he learnt to stand out of his shadow and find his own character. It's the same story of Rita from 'The God Complex', who's both training to be a doctor and sharper and brighter than everyone else in the room. Or Dr Constantine from 'The Empty Child', who like Nine, is a doctor that's lost everything but just has to keep going. Even Professors Yana ('Utopia') and Palmer ('Hide') relate to the Doctor; the former is a gifted physician trying to help human kind and the latter is a scarred war veteran investigating strange goings on with a younger, female companion. If you ever want to show contrast/similarity between the Doctor and a new character, enrol them in the NHS and give them some dialogue with the Oncoming Storm. It's the oldest trick in Who's book, so don't be surprised if you see it a dozen more times before the 60th anniversary.