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With Karra's death came a rage. Ace couldn't explain it. She couldn't get over it. She was angry at the world for taking such a beautiful soul away. She was angry at the world for all of the injustice that went with it.
She left the Doctor, his positive world view too much for her to handle any longer. She stayed on the run, traveling around the world and never heading back to Perivale. If she went back to becoming an arsonist, that was her story to tell.
The Doctor attempted to keep an eye on her, but failed often.