Vastra

Vastra was a female Silurian warrior who shared a house at 13 Paternoster Row with a human girl, Jenny Flint, in Victorian London. They shared an intimate relationship.

Biography
Vastra 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". After Vastra had slaughtered at least five commuters, the Doctor convinced her not to give in to rage.

At the behest of Scotland Yard, she tracked down and ate Jack the Ripper. Immediately after, she was summoned by the Eleventh Doctor to help rescue Amy Pond and her baby from Demon's Run. Jenny and she arrived at Demon's Run with an army of Silurian soldiers. They both survived the battle with the Headless monks, and displayed skilled swordsmanship while participating. She called the Doctor 'old friend,' and judging by how well she seemed to know him and the fact that she said she owed the Doctor a "very old debt", they must have known each other for a long time. He had told her about Time Lords; she referred to regeneration, the Untempered Schism and the time vortex. (TV: A Good Man Goes to War)

Vastra, alongside Jenny and the Sontaran Strax, later summoned the Doctor for help. The Doctor refused, claiming he had "retired." (TV: The Great Detective)

Personality
Although Vastra enjoyed eating humans, she had a romantic relationship with Jenny. Vastra could control her darker urges, and she spared two men when she and Jenny were helping the Doctor despite commenting 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 controlled her anger and did not seek revenge on the humans who had awakened her.

Vastra was a fierce warrior who wielded two swords in combat. She held her own against a group of headless monks, surviving without any apparent injury. She was 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 well.

While she shared some of the prejudices of many Silurians against humans, saying all "Mammals" looked alike and threatening to eat one, Vastra 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 the infant Melody. (TV: A Good Man Goes to War)

Spin-off?
Since their appearance, Vastra and Jenny have become quite popular among fans, with many wishing for a spin-off series of their adventures. Steven Moffat addressed this, saying he considered a spin-off when he created the two characters, but wouldn't have the time, though he would like to revisit them.

The Brilliant Book 2012
According to The Brilliant Book 2012, a book that contains non-narrative-based information;
 * She made a home in London, at first making a living as the main act in Henry Gordon Jago's "Monstre Gathering" (to the Doctor's disapproval) and then as a masked bank robber. After finding lodgings above a gin palace in Cheapside, she came across Jenny, a match girl who was being molested by a Chinese gang. She saved Jenny from the Tongs and subsequently began a relationship with her.
 * In 1881, Vastra visited Egypt and was mistaken for a god.
 * At some unknown point in Japan's history, she and Jenny battled some Cybermen.
 * Following their return to their own time, Vastra and Jenny were added to the Papal Mainframe "most wanted" list.