The Ninth Doctor
"I'm left travelling on my own 'cause there's no one else."
When Rose first meets the Doctor, he is a man in emotional turmoil following the events of the Time War, in which he was forced to destroy his whole planet and race in order to save the entire universe from the Daleks.
With the help of his new companion, he learns to deal with his guilt, and in time he becomes more patient and less cold-hearted.
In return, he helps Rose see that there's more to life than just a 9-5 routine, and together they journey across the stars, meeting the likes of Charles Dickens, Slitheen and gas-masked zombies along the way.
When the Doctor sacrifices himself to save Rose from the powers of the time vortex, he faces his latest regeneration with bravery rather than fear. As the process begins, he thanks Rose for the time they spent together, and, with his last breath, tells her that she had been fantastic, noting that, despite everything he had been through, so had he.
With that, he transformed before her eyes - a man now healed from the devastating events of his past.