The Eight was the main alias used by the eighth incarnation of a Renegade Time Lord who suffered from regenerative dissonance. Unlike his other incarnations, he attempted to be a "good man". Using meditation, he was able to suppress the voices of his previous incarnations.
Living as Father Octavian Edit
He became involved with the Time Lords' (and his) future, attempting to stop the Eighth Doctor from assembling the Doomsday Chronometer. He linked minds with the Doctor and "Sister Cantica" to stop the Clocksmith. After he was killed by the Clocksmith, the Eight retreated into the Doctor's TARDIS and regenerated. When the regeneration occurred, he heard the voices of his previous selves, who urged him to regenerate and also taunted him for not being one of them, while the Six wanted to return so he could kill people again. (AUDIO: The Eighth Piece / The Doomsday Chronometer)
When the Eleven made a deal with the citizens of the Dark Universe to control the universe, he transferred his other selves into robot bodies. The Eight persona was reduced to the role of cleaning the Eleven's palaces, but this put him in a position to help Ace, Cardinal Ollistra and Captain Rasmus when they came to defeat the Eleven, the Eight using his cleaning solution to neutralise the other soldiers and lead them to the Dark Gate so that they could convince the Dark Citizens to withdraw (AUDIO: Dark Universe).
As a thriving persona inside the Eleven's consciousness, he successfully took control of his physical body and told the Doctor about Padrac's plans to use the Resonance Engine as a means to amplify Caleera's powers. (AUDIO: Stop the Clock)
When the Eleven acted to undermine the Nine's plan to use the Ravenous to kill the Doctor, Liv and Helen, the Eight assumed that he had "seen the light", but the Eleven coldly corrected his predecessor that he was only sparing them now in order to deliver a more devastating blow later. (AUDIO: The Odds Against)
When the Eleven was pretending to have been 'cured' of his condition after an attack by the Ravenous, he occasionally allowed the Eight out to give the impression that Eight was helping him keep the others in check, the other nine personas threatening to bury the Eight in their subconscious for good if he didn't go along with the lie (AUDIO: Whisper, Planet of Dust)
While the Twelve discussed her previous selves with the Doctor, who referred to the previous personas as "a mad stew of voices and personalities" who were all "mad and homicidal", she thought the Doctor was being unfair on the Eight, whom she described as "sweetness itself". The Doctor then conceded, and described him as "the exception that proved the rule". (AUDIO: Planet of the Ogrons)