Anna

Annastasia Amelia Andrai (usually known as Anna) was a companion of the  The Doctor briefly during the last days of the his  eleventh incarnation and for several adventures in his  twelfth incarnation.

Biography
Anna, properly known as Annastasia Amelia Andrai, was a young Human colonist on the planet Luwei in the 33rd century, and a delegate of the High Alliance, a major alien organization responsible for much of the law in the 33rd century. She met the Doctor when he was being held on trial by the High Alliance for unknown reasons, and promised to set him free, much to his confusion. (The Trial of the Doctor)

In truth Anna had been visited by an unknown woman in her youth telling her that an old friend would be in danger soon, and that he would need Anna's help. Anna had grown up believing that this man would one day appear in her life, and she promised to help him when that time arose. (Arthemos)

When the Alliance sentenced the Doctor to death Anna, with the help of her friend Jak, smuggled a key into his cell and helped him escape. They managed to get to the chamber holding the Doctor's TARDIS, but in the ensuing crossfire Jak was killed and the Doctor fatally injured. He collapses in the TARDIS with Anna beside him, and tells her to go. He promises her that he will find her again, though he will have a different face. (The Trial of the Doctor)

The next time Anna sees the Doctor is nearly two years later, when he arrives on Luwei in his twelfth incarnation. She is overjoyed to see him, though initially uncomfortable with his new appearance. The Doctor offers her a journey in the TARDIS, which she happily accepts. They journey to the Arthemos in the year 4444, a massive space station wherein humanity and the Chylor are locked in a vicious stalemate that has been going on for almost one hundred and twenty years. The Doctor and Anna help humanity drive back the Chylor. (Arthemos)

After this the Doctor returns Anna to her home time and promises to see her again, though he does not know when.

Behind the Scenes

 * Anna has been speculated to appear in the eleventh season of Doctor Who.