Davros, why must your children be so detail oriented?
Rusty is an unusual case amongst the ultimate racists. He was an average Dalek until a battle left him adrift in space. His power source was cracked and leaked radiation. It broke a piece of programming that prevented Rusty from feeling anything but hatred against non-Daleks. Seeing the birth of a star made Rusty realize that life will always come back despite the Daleks’ best efforts.
He was picked up by a ship and made the crew an offer; fix him up and he will help the humans fight the Daleks. The crew agreed and set to work. Sadly, the work damaged the creature inside and so the Doctor had to intervene. The Doctor, Clara, and a couple of crew members shrunk down and went inside the casing to fix Rusty. They fixed the crack, but without the radiation affecting him, Rusty fell back on his old programming.
Clara convinced the Doctor to restore Rusty’s memory of the star’s birth and linked his mind with the creature. The Dalek saw what made up the Doctor, but latched onto his hatred of the Daleks and so went on a murderous rampage against the Dalek rescue party.
Over time, he joined a newly formed Unified Nebular Intelligence Task force, UNIT. He had made friends with an old member and a figurehead called Brigadier Alistair Gordon Lethbridge-Stewart; a formerly dead man now restored as a Cyberman. Now Rusty acts as a Colonel and fights alongside the Brigadier.
A recent adventure resulted in his meeting of Princess Megumi and her Vortex Riders, from other universes. He, along with the Brigadier, joined the Vortex Riders as Tech Support and manage the gateway on Vorton.