Meta-Crisis Doctor

The "Clone" Doctor was created as a result of regenerative energies present within The Doctor's severed hand. When Donna Noble touched the casing of the severed Hand, it broke as it took in the Human DNA and created a Time Lord/Human hybrid that possessed one heart and aged as a Human did. The new Doctor also contained parts of Donna's personality and mannerisms but mixed with his own intelligence. He still has the wit and humor of the original Doctor, and also his abhorance to what the Daleks are doing to the humans, and plan on doing to the rest of the universes. But while the original Tenth Doctor could never again just completely destroy the new Dalek Empire, the new Doctor can and does. Though this saves all of creation, the original Doctor views this as simply wrong.

The new Doctor had caused the genocide of the Daleks, something the original Doctor disapproves, even when it comes to the Daleks. Because of this, the original Doctor decides he must stay on the parallel earth known as Pete's World. The new Doctor, being born at a time of war, is almost like his ninth incarnation. The original Doctor tells Rose Tyler that she is the only one who can make him a better man, as she already has before.

The new Doctor has one major difference than his original self. As he is part human, he only has one heart and can age and die without the ability to regenarate. Being part human also gives the new Doctor one advantage over the original... He has the ability to say and do things that the full Time Lord cannot. He tells Rose that he IS the Doctor, with the same thoughts and the same memories and feels the same things, and that as a part human wants to spend his life with her and grow old together. When asked by Rose what he would have said to her that day on the beach of Bad Wolf Bay, he is able to whisper to her how he felt then and how he still feels now. When Rose hears these words and looks up at him, she knows that, though not the original, he is still her Doctor and the two are able to finally have their loving and passionate (and long awaited) kiss.

When the original Doctor leaves, the new Doctor sees that Rose's heart is breaking for that version of him. But he takes her hand, like he has thousands of times before, and she looks up at her Doctor and loves him just as much. This new Doctor can give himself to her completely, and vice versa... The former Time Lord and the former shop girl begin anew. The soulmates finally able to be together. Their adventure is only just begining...

Behind the scenes

 * Although he the same appearance, memories, and basic personality of the Tenth Doctor, the new Doctor also exhibits several personality changes, in particular based upon that of Donna Noble. It is left deliberately ambiguous as to whether the new Doctor is actually a new incarnation of the original Doctor (if not strictly speaking a new regeneration) or a copy.
 * In the Doctor Who Confidential installment, "End of an Era", executive producer Julie Gardner confirms that the new Doctor does indeed say "I love you" when he whispers in Rose's ear.