Melody Pond (Prequel to The Impossible Astronaut)

The unnamed girl was a mysterious being who, according to River Song, appeared to be human. However, she possessed superhuman strength and the ability to regenerate, suggesting a biological connection to the Time Lords.

Biography
The girl contacted Richard Nixon for assistance, but he did not know who was calling him. She told him about the "Monsters", and how she feared them. (WC: Prequel (The Impossible Astronaut))

When Nixon asked where or who she was, the girl told him "Jefferson Adams Hamilton", leading him to believe her to be a boy. However, the Doctor realised these were the names of streets, the only three by those names intersecting in Florida, and probably all of America.

She watched from a distance when the Doctor arrived to find her. Whilst wearing what appeared to be an astronaut's suit, the girl eventually approached the Doctor and Amy Pond, the latter firing a gun at her, as she initially believed that the girl was the assassin of the Doctor, 200 years in his future, who wore a similar, if not identical, astronaut suit to her. (DW: The Impossible Astronaut)

Three months later she again encountered Amy, who had only punctured the suit's visor, in what appeared to be her bedroom at the Graystark Hall children's home. The room contained a number photos of the girl, including one of Amy holding an infant. The Silence then appeared in the room behind the girl and took Amy hostage.

The girl was able to free herself from the suit, which had been fitted with alien technology to act as a life support system, displaying enormous strength. She then ran from the scene after spying on the Doctor, Canton, River and Rory rushing to help Amy.

Six months later, her deteriorating health caused her to regenerate in a New York alleyway. (DW: Day of the Moon)

Behind the scenes

 * The episode A Good Man Goes to War revealed that River Song (born Melody Pond) was actually the daughter of Amy Pond and Rory Williams, and possessed Time Lord DNA as a result of being conceived whilst the TARDIS was travelling in the time vortex. It was implied in the episode, through the use of flashbacks and speculation by characters that Melody might be able to regenerate, that the little girl is Melody/River. This was proven true in Let's Kill Hitler.