Vastra

Vastra was a female Silurian warrior who shared a house with a human woman, Jenny, in Victorian London, with whom she shared an intimate relationship.

She was one of many people who owed the Doctor a debt: she had awoken during the construction of part of the London Underground and had wanted to take revenge on innocent tunnel workers for the deaths of her "sisters"; the Doctor convinced her not to give in to rage. Vastra instead made a home in London, disguising her Silurian form to most people and acting as a freelance adventuress.

After eating Jack the Ripper, she was summoned by the Doctor to help in the mission to rescue Amy Pond and her baby at Demon's Run. She and Jenny arrived at Demons Run along with an army of Silurian soldiers. They both survived the battle with the Headless monks. It was implied she had known the Doctor for some time as she referred to him as an old friend. The Doctor had also informed her about Time Lords as she referred to regeneration, the Untempered Schism and the time vortex. (DW: A Good Man Goes to War)

Personality
Although Vastra enjoyed eating humans when she had cause to do so, she also had a romantic relationship with Jenny. Vastra was shown to be able to control her darker urges as she spared the lives of two men when she and Jenny were helping the Doctor despite commenting that she was hungry. She poisoned one of them with her venom, but this was only to stop him from calling for help. When her sisters died in hibernation, Vastra was enraged, but with the Doctor's help she was able to control her anger and did not seek revenge on the humans who awakened her.

Vastra was a fierce warrior who wielded two swords in combat and held her own extremely well against a group of Headless monks, surviving without any apparent injury. She was also very loyal and protective, ordering her allies to protect Melody Pond at all costs. Vastra was an old friend of the Doctor and seemed to know him quite well.

Other information

 * Vastra appeared to know the Doctor very well, referring to him as an old friend. She knew the Doctor well enough that when she asked him how Melody could have become a Time Lord, she attempted to be delicate because she knew how he could blush. Also when the Doctor's anger got the better of him she repeated the words that he had told her when he stopped her from killing the tunnel diggers. Vastra was also quite knowledgeable about the Time Lords, knowing about their ability to regenerate. She also had knowledge of the Untempered Schism and the Time Vortex, which showed a considerable knowledge of time itself. Vastra suspected that the reason Melody Pond was part Time Lady was because she had been conceived in the TARDIS while it was travelling through the Time Vortex.
 * While she shared some of the prejudices that many members of the Silurian race have against humans, stating that all "Mammals" looked alike and threatening to eat one, she was much more tolerant and accepting of humans than many of her species, apologising for her remarks about humans to Jenny and showing concern for the safety of Melody.

Behind the scenes

 * Since their appearance, Vastra and Jenny have become quite popular among the fandom, with fans wishing for a spin-off series to be made based on their adventures. Steven Moffat addressed this, saying that he considered making a spin-off when he created the two characters, but wouldn't have the time, though he said he would like to revisit them.