Kotris

Kotris was an incarnation of the Time Lord Straxus who worked with the Daleks.

Biography
Straxus became fed up with the morally questionable actions of himself and his people, and allied himself with the Dalek Time Controller. The Dalek Time Controller helped him rewrite his biodata to remove his Time Lord heritage, so that when they retroactively destroyed the Time Lords the new Straxus — now calling himself Kotris, an old family name of his — would survive. Kotris knew that as Straxus he had attempted to avoid regenerating, and sent a temporal drone to prevent Straxus killing himself.

Kotris and the Time Controller planned to incubate "retro-genitor particles" in a human named Molly O'Sullivan. They sent Daleks after her and the Eighth Doctor, forcing them to spend time together and allowing the retro-genitor particles to latch onto the Doctor's Gallifreyan DNA. The particles were then intended to follow that DNA back through history, undoing not just the Doctor's life, but all of Gallifrey's past.

An earlier incarnation of Straxus modified the retro-genitor particles implanted in Molly to target the Daleks instead of the Time Lords, but in a confrontation with Kotris and the Dalek Time Controller, the Time Controller killed him. This meant he could never become Kotris, and the scheme was entirely undone. (AUDIO: The Great War, Fugitives, Tangled Web, X and the Daleks, The Death of Hope)