Due to a malfunctioning repair subroutine, Green-8 developed sentience at some point in his existence, but lacked the ability to explore that sentience as he had nobody to communicate with; his fellow Gardeners lacked his sentience, the "Scavengers" of the planet ran away whenever they saw him, and the "Lords" lacked the ability to really register his presence. Green-8 remained trapped in this status quo until he encountered the Doctor after he had attempted to escape from the other Gardeners, Green-8 disobeying the order to apprehend the Doctor in the hope of having a conversation with him.
Quickly establishing Green-8's sentience, the Doctor took it upon himself to help Green-8 explore the mysteries of Esselven Minor and his own sentience, helping Green-8 free some scavengers from the captivity of the Red robots — recognising that their own programmings had developed a malicious glitch that caused them to see Green-8 as beneath them — while Green-8 helped him develop a disguise as another Green sector robot to pass through the Gardens unmolested. Arriving at the palace, the Doctor learned that the "Lords" were actually just holograms of a story that had started long ago and been left on a loop ever since- which prompted Green-8 to grasp the concept of humour for the first time as he pondered the ridiculousness of his fellow robots supplying holograms with food for centuries- while the "Scavengers" were the true humans who had been driven out after an accident and kept away ever since due to a programming fault.
With the aid of Green-8 and Oralissa, a sentient hologram, the Doctor was able to set up an elaborate deception to drive conqueror Glavis Judd and journalist Dexel Dynes away from Esselven Minor, Green-8 subsequently remaining to help the scavengers re-learn how to be the Lords and rulers they had to be for their homeworld. He was later shown on Esselven, now dressed in robes and going by the name Greeneight, King Kel the Third referring to Greeneight as his "old conscience" due to the morality Greeneight had developed thanks to his lessons from the Doctor. (PROSE: Palace of the Red Sun)