Jenny (The Doctor's Daughter)

Jenny was the "daughter" of the Tenth Doctor, artificially created from his genes.

Creation
During a short war waged on the planet Messaline between humans and Hath, both sides used progenation machines to instantly create mentally programmed young adults from DNA of a single parent. When the Tenth Doctor, Donna Noble and Martha Jones, arrived on Messaline, Jenny was made from him against his will from a skin sample, moments before an ambush by Hath. (DW: The Doctor's Daughter)

With the Doctor
The then nameless Jenny pushed a button which triggered an explosion. The tunnel collapsed, cutting the group off from the Hath and Martha. Shortly after, Jenny was named by the Doctor's companion Donna Noble from his description of her as a 'generated anomaly'.

Jenny's commander, Cobb, had the Doctor, Donna, and Jenny locked up. Donna proved to the Doctor that Jenny indeed his daughter by listening to Jenny's heartbeat. Like the Doctor, she had two hearts. However, the Doctor insisted that she was nothing more but an "echo", and that a "real" Time Lord was "so much more".

Jenny helped the others escape by kissing Cline and stealing his pistol. The Doctor found his daughter very capable, though she was inclined towards violence. He later convinced her not to kill Cobb.

As the three of them made their way towards the Source, which Cobb and the Hath both sought, Jenny spoke with the Doctor about the possibility of travelling with him, and he told her that he would never leave her. The Doctor told the respective parties to end the war, whereupon General Cobb aimed his gun and shot at him. Jenny jumped in the way of the bullet. She was shot through one of her hearts, and died, but did not regenerate. (DW: The Doctor's Daughter)

While laying in state, Jenny's body was revived by the gaseous compound from the Source. (REF: The Time Traveller's Almanac) She stole a shuttlecraft and left Messaline. When asked where she was going, she restated Donna's earlier description of the Doctor's life. "Oh, I've got the whole universe: planets to save, civilisations to rescue, creatures to defeat -- and an awful lot of running to do." (DW: The Doctor's Daughter)

Legacy
Davros tried to manipulate the Doctor when he moved the Earth to the Medusa Cascade by asking the Doctor how many people had died in his name. Jenny was one of the people he thought of, as he was unaware of her revival. (DW: Journey's End)

Personality
Jenny showed a lot of the brilliance, lust for life, and determination of her father. She was also flirtatious. Though programming had made her military-minded and goal-oriented, she had too much of her father in her blood. She soon adopted the Doctor's values and principles, though she also challenged him. (DW: The Doctor's Daughter)

Inherited characteristics
Being biologically Gallifreyan, Jenny had two hearts, as well as reflexes, precision timing, and acrobatic ability far beyond that of an average human (DW: The Doctor's Daughter). Jenny's revival however was not due to regeneration, but the hidden properties of the Source (REF: The Time Traveller's Almanac).

Other information
The presence of Jenny drew the Doctor's TARDIS to Messaline, paradoxically, before her creation, so that she, in effect, caused herself to exist. (DW: The Doctor's Daughter) However, this may be due to the TARDIS being old and faulty, so it got there earlier than the time to which it was being summoned.

As the Doctor was Jenny's father, many of her relatives are unknown. She was the Doctor's youngest child as he stated that he had fathered children and had a family before, making Jenny their sister on some level (or more accurately their half-sister as, assuming the Doctor's other children were born naturally, they would have the same father as Jenny, but a different mother, since Jenny did not actually have a mother). (DW: The Empty Child, DW: Fear Her, DW: The Doctor's Daughter) Susan Foreman, the Doctor's granddaughter who travelled with him during his first incarnation and was the only other relative other than Jenny to appear on screen, was technically Jenny's niece, (DW: An Unearthly Child, DW: The Empty Child) assuming Susan was truly the Doctor's biological granddaughter. (NA: Lungbarrow) River Song, who was married to the Eleventh Doctor, was technically Jenny's stepmother. (DW: The Wedding of River Song)

Behind the scenes

 * Although not considered a companion of the Doctor's, Jenny can be, at least, considered a "companion presumptive". It is clear, once the Doctor accepted her as his daughter, that she would travel with him and Donna. Her "death" ended these plans. This places Jenny in the same category as Astrid Peth, Lynda Moss, Madame de Pompadour, and Rita, who were all invited to travel with the Doctor, accepted the invitation, yet died before they were able to do so.
 * Jenny was played by Georgia Moffett, the daughter of Peter Davison, who played the Fifth Doctor. Moffett had auditioned for several other roles on the series, including Rose Tyler and had also performed in a Doctor Who Big Finish Productions audio drama alongside her father in 2000.
 * Steven Moffat is credited with suggesting that Jenny be brought back to life at the end of the episode. This led to rumours that Georgia Moffett was in the running to return as a companion for the Eleventh Doctor. Ultimately, Karen Gillan was cast in this role as Amy Pond.
 * Although The Sun, a UK tabloid, speculated that Georgia Moffet would appear as Jenny in one of the 2009 specials, she did not make an appearance. No further appearances of the character have been announced. Moffett did return to the franchise in other roles, providing the voice of an unrelated character named Cassie Rice in the animated serial Dreamland, and continuing to do voice work for Big Finish Productions in 2010.
 * Actress Georgia Moffett played the Doctor's daughter, is the daughter of Fifth Doctor actor Peter Davison, and recently gave birth to the daughter of Tenth Doctor actor, David Tennant.