Idris

Idris was a Human-appearing woman who was used as a host for the Doctor's TARDIS.

As Idris
The details of Idris' early life remain unknown. On the asteroid inhabited by House, she lived with fellow humans Uncle and Auntie and an Ood called Nephew. Unlike her fellow humans, she was never 'repaired' by House. Nephew (under House's orders) later drained Idris of her mind and soul in preparation for the arrival of a Time Lord, the Doctor. House then removed the matrix from the Doctor's TARDIS and placed it inside Idris' body. At this point, though her body was still intact, Idris essentially died. (DW: The Doctor's Wife)

As the TARDIS
When the Doctor, along with Amy Pond and Rory Williams, arrived, they were spotted by Idris, who now housed the soul of the Doctor's TARDIS. When told to get some rest, Idris asked if there was an off switch and instantly fell asleep, before being carried away by Nephew. She was locked away in a small cell. The Doctor released her and, with the Doctor, watched as Auntie and Uncle died. When the Doctor asked her if she had a name of her own, she responded that the Doctor had given her the name "Sexy" when in private, embarassing him a bit. Idris realised that her body was too weak to house the TARDIS' matrix, and she had under an hour left to live.

Eventually they agreed to rebuild a makeshift TARDIS from a "graveyard" of the vehicles, while they bonded. Idris then infused the mechanical bits with a part of the TARDIS' inherent energy to allow them to follow House. The Doctor had Idris send a telepathic message to Amy with details on how to get to a backup control-room and raise the shield; having been told by the Doctor to send a message to 'the pretty one', Idris sent the message to Rory.

When they got the shield down, the new console landed on Nephew and blasted him into atoms. The Doctor quickly introduced Amy and Rory to Idris, who introduced herself as 'Sexy'. Idris collapsed, close to the end of her life. The Doctor tricked House into transporting them into the main control room. The Doctor pretended to congratulate House for defeating them, while Idris, about to die, whispered "the only water in the forest is the river" into Rory's ear. The matrix was released inside the control room and merged back with the TARDIS. It proceeded to purge House from the TARDIS in a battle of souls that House couldn't win. With the last of her energy, the matrix used Idris's body to tell the Doctor that, although she would always be there for him, this was the only occasion when they could talk, using her last moments with a voice to say the word she never got to say to him when he first left Gallifrey: "Hello. Hello, Doctor. It's so very, very nice to meet you." (DW: The Doctor's Wife).

Behind the scenes

 * On his blog, Neil Gaiman stated that Idris arrived on the asteriod because she came through the Rift. He also said that she was dressed in a Victorian dress because she had been to a Victorian party.