Meta-Crisis Doctor

"No, no, no, no, no. I'm unique. Never been another like me."

- Meta-Crisis Tenth Doctor.

The half-Human Meta-Crisis Tenth Doctor was created as a result of an instantaneous biological meta crisis. Regenerative energies present within The Doctor's severed hand combined with the Human DNA of Donna Noble when she touched the hand's casing. The force of the reaction shattered the case allowing the energy to regenerate the entire missing body of the Doctor creating a Time Lord/Human hybrid, a hybrid that possessed one heart; aged as Humans do and with no regenerative ability to avoid death. As a result of growing partially from Donna's DNA the new Doctor picked up parts of Donna's mannerisms but maintained that he possessed the same memories, thoughts and feelings of the original Doctor--essentially they were the same man with physiological differences. However, the original Doctor is quick to point out his new double was born in battle, full of blood, anger and revenge which enabled him to commit genocide against the Daleks; an act the original Doctor greatly condemns after the events of the Last Great Time War.

Feeling his new half-Human counterpart is too dangerous to be left to his own devices, the Doctor entrusts him to Rose Tyler taking the two of them back to Bad Wolf Bay in the parallel universe dubbed Pete's World. The Doctor tells Rose that his half-Human self needs her, as he is angry and vengeful like he himself once was when he met Rose as the Ninth Doctor. He tells her that she made him better, and now she can do the same for his other half-Human self. She objects to this at first, insisting the new Doctor isn't really him despite the Doctor's assurance that they are the same man and the half-Human Doctor offering to spend the rest of his life with her. (Their interest in one another is shown briefly in several parts/shots of "Journey's End," with Rose and the "new Doctor" clearly are laughing, flirting, staring at one another and still seemingly having the same feelings they originally held for one another.)

When she feels guilty/sad at the possible loss of the original Doctor, Rose decides to then asks both Doctors what the last thing they said to her was when they were originally standing on Bad Wolf Bay (DW: Doomsday). The Doctor is unable to give her a direct answer, only allowing himself to hint: "Does it really need saying?" (Hence, showing that the lack of "human" in him keeps him from portraying his true feelings, where as the half-human Doctor can.) At this point she turns to the half-Human Doctor and poses the same question, to which he steps forward and whispers the  answer his other self could not say in her ear.

Seemingly overcome with the revelation of his answer and the happiness and completeness they are finally able to share, Rose grabs the lapels of the half-Human Doctor and pulls him into a passionate kiss, which he happily and strongly reciprocates. The Doctor looks on sadly, yet stoicly, for a brief moment, before returning with Donna to the TARDIS (knowing full well what is about to happen to Donna) and leaving the parallel world. As the doors of the TARDIS slam shut, Rose and the half-Human Doctor break from their kiss, but it is too late for them to say their final goodbye. The half-Human Doctor takes Rose's hand as they watch the TARDIS dematerialise for what would appear to be the last time.

Grow Your Own TARDIS
In the original script the Doctor was meant to give a TARDIS coral to the half-Human Doctor under the assumption that a whole new TARDIS could be grown from it, but it was later cut. However, the canonicity of this event is now somewhat ambiguous.
 * This was mentioned in "The Doctor's Data" section of a Doctor Who Adventures magazine.
 * It was mentioned in the Fact File for the episode on the official website, which stated that it was in the original script but later removed.
 * In addition, if one looks carefully at the 4th cover of the August 2008 Doctor Who Magazine, featuring Rose, Mickey, Jackie, and the Meta-Crisis 10th Doctor, it appears that, although covered up by actors' names, that the duplicate Doctor seems to be holding something. This could possibly be a piece of TARDIS.
 * Inside the magazine, there is an excerpt of script with a statement that this part of the scene made it all the way to the last cut, but the producers decided that it was just complicating the scene too much. However, the scene was filmed, and it will be included in the Series 4 Box Set, so, the magazine quotes Russell T. Davies, it is perfectly fine to assume that the Meta-Crisis Tenth Doctor and Rose Tyler have a piece of TARDIS coral, which they can use to grow their own TARDIS.
 * It should be noted, however, that a Radio Times article suggested that it would take Jack Harkness several hundred years before he could "begin the carving process" of the TARDIS he is growing in the Torchwood Hub. However, it is possible that the coral given to the Meta-Crisis Tenth Doctor and Rose by the Tenth Doctor will grow more efficiently than the coral Jack was using, the origin of which is unclear.
 * A line cut from the scene featured Donna suggesting a way to speed up the coral's growing process.

Behind the scenes

 * Although he has 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 instalment, "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.
 * On screen, this character is only ever referred to as the Doctor. To avoid confusion, fans have taken to referring to him by names such as the Clone Doctor, New Doctor, Meta-Crisis Doctor, Doctor Two or Doctor 10.5, though none of these names are considered official, particularly references to him as the 11th Doctor, although David Tennant, in Doctor Who Confidential, acknowledges that this is expected to be a topic of much debate as time goes on.