Oswin Oswald was a human serving as a Junior Entertainment Manager onboard the starliner Alaska when it crashed into the Dalek Asylum.
Biography
Oswald initially tried to escape the Asylum but was found and converted into a Dalek. Unlike other members of the Alaska, who were partially converted by the nanocloud into Dalek puppets, she was fully converted by the Dalek inmates, due to her exceptionally high intelligence. Due to the horror of what had happened to her, Oswin created a dream world for her human mind to dwell in to forget the truth. Under this life, she assumed that she was surviving on soufflés and hacked into Dalek systems.
A year later, she aided the Eleventh Doctor, Amy Pond and Rory Williams in finding safe passage, wiping the memory of the Doctor from the Daleks' Pathweb at one point to save him. The Doctor attempted to rescue her, until he discovered she was a Dalek. The Doctor explained this to Oswin, who first nearly gave in to the Dalek conditioning by attempting to exterminate the Doctor, then broke down over what she had become.
Oswin lowered the Asylum's defences so the Parliament of the Daleks could destroy the planet, and presumably died when the Daleks blew up the Asylum. When Amy, Rory and the Doctor escaped back to the Parliament, they found out that Oswin had erased the Doctor from the memories of all the Daleks in the Parliament as well as the Dalek inmates in the Asylum. (DW: Asylum of the Daleks)
Personality
Oswin had a flirtatious and feisty personality, and cheerfully flirted with both the Doctor and Rory. Following her conversion, she was unable to accept the horror of what she'd become, and deluded herself into thinking that she was still human. Oswin had a strong-willed nature, and even after her conversion, rejected mental conditioning in favour of retaining her humanity. Notably, she was the only Dalek that the Eleventh Doctor displayed any compassion or pity for.
Oswin was selfless, allowing the Doctor to complete his mission to blow up the planet and insisting that he leave her behind and escape with Amy and Rory. She was also extremely intelligent, and it was because of her intelligence that she was turned into a Dalek. She had knowledge of Daleks and their technology that exceeded even the Doctor's, because she had been converted into a Dalek herself. (DW: Asylum of the Daleks)
Behind the Scenes
Nicholas Briggs, the usual Dalek voice artist, voiced the dialogue spoken by Oswin as a Dalek.