Boris (Dead of Winter)

Prince Boris was a Russian noble during the mid-to-late 1700s. He was sent to St Christophe when he contracted tuberculosis, but was eventually cured by the Familiar. Unfortunately, he had long planned to take over the world, but he had never seen any point in carrying out his plans, as he was going to die anyway. With his days no longer numbered, he formed a link with the Familiar hive mind, allowing him to take control of it and use the normally-benevolent creature for malicious purposes; namely, possessing his fellow patients, making them into his own personal army. His plan was eventually defeated when the The Sea made the mistake of scanning the Eleventh Doctor's mind; the accumulated pain he had felt over the centuries was too much for the Familiar, and it was destroyed. With his source of power gone, and its cure rapidly fading, Prince Boris was left to live out the last days of his life in St Christophe. (NSA: Dead of Winter)

Personality
When the Doctor and his companions first encountered him, Boris initially came across as bored and apathetic about almost everything - but a rather pleasant individual. However, when his health was restored, he showed his true colours as an amoral military mastermind, willing to manipulate whomever he wanted to achieve his ends.