Rusty (Into the Dalek)

"Rusty" was the name given by the Twelfth Doctor to a Dalek that was captured by the forces of Combined Galactic Resistance. Voicing its belief that the Daleks needed to be exterminated, "Rusty" was investigated by the Doctor, only to see if it was possible to exist a 'good Dalek'.

Biography
Seeing the birth of a star triggered a realisation in Rusty of the inevitability of life returning. 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 evil 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 good again and had Clara reactivate his 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, but the beauty Rusty saw was in the destruction of the New Dalek Empire by the Meta-Crisis Tenth Doctor who Rusty saw as evil. (TV: Journey's End)

Rusty then went on a rampage against his own kind, slaughtering the Daleks attacking the Aristotle and calling for a retreat by the saucer, claiming the ship's self-destruct had been activated. Rusty left with the Daleks to continue his efforts against them, telling the Doctor that there are no good Daleks but that the Doctor would make a good one himself. (TV: Into the Dalek)

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 (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 shared by both Rusty and the Doctor. (TV: Into the Dalek)