"Rusty (Into the Dalek)"
"Rusty" was the name given by the Twelfth Doctor to a Dalek captured by the forces of Combined Galactic Resistance. Intrigued by Rusty's desire to exterminate the Daleks, the Doctor decided to explore the possibility for a "moral Dalek".
Biography
Seeing the birth of a star triggered a realisation in Rusty of the inevitability of life returning despite the efforts of the Daleks. The Doctor, Clara Oswald, Journey Blue, Ross and Gretchen Carlisle were shrunk and ventured into Rusty to try to fix the damage that was killing him. Using his sonic screwdriver, the Doctor repaired a crack in Rusty's power source that was leaking radiation and killing him.
However, without the radiation affecting him, Rusty turned immoral again, restored to full power and went on a rampage, killing several soldiers and sending a distress call that attracted a nearby Dalek flying saucer. Upon urging from Clara, the Doctor decided to try to turn Rusty moral again. He had Clara reactivate the Dalek's suppressed memories so he could see the birth of the star again and be reminded of the universe's beauty.
Clara succeeded and the Doctor telepathically linked with Rusty to show him more from inside his own mind. At first it seemed the Doctor had succeeded, as Rusty again saw beauty and divinity. However, Rusty also saw the Doctor's hatred of the Daleks, which resonated even more strongly with him. Rusty saw the Daleks as an evil which must be, in true Dalek fashion, exterminated.
Rusty then went on a rampage against his own kind, slaughtering the Daleks attacking the Aristotle and fooling the saucer into retreating by falsely telling the other Daleks that the Aristotle's self-destruct had been activated. Rusty left with the Daleks to continue his efforts against them, telling the Doctor that although Rusty himself was not a "good Dalek", the Doctor was a good Dalek. (TV: Into the Dalek)
The Doctor remembered what Rusty had said to him when Missy offered him an army of Cybermen for his birthday that he could use to rule the universe. (TV: Death in Heaven)
Memories
While he presumably did not participate in these events, Rusty had access to the memories of Henry Van Statten's "Metaltron" exterminating Bywater as it escaped the Vault in the Battle of GeoComTex (TV: Dalek) and Davros's New Dalek Empire attacking the Valiant during the 21st century Dalek invasion of Earth. (TV: The Stolen Earth) The destruction of the Daleks, the fleet and the Crucible at the failure of said invasion (TV: Journey's End) were memories Rusty saw while telepathically connected to the Doctor. (TV: Into the Dalek) This may all be a result of Rusty's connection to the Pathweb (TV: Asylum of the Daleks).
Rusty did not seem to recognise the Doctor in his new incarnation, calling him Doctor only because he believed him to be a healer sent to fix the Dalek. It is unclear whether Rusty knew of the Doctor's history with the Daleks prior to being telepathically linked with him. (TV: Into the Dalek)
