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If I aggressively take out the soot, it will only kill her. I have already analyzed it and it will end up tearing her apart from the inside out, this is because the soot has stuck to her flesh, the interior of her blood vessels, even her heart and her lungs, before I can even take it out, she would be dying due to bleeding and I wouldn't be able to heal so many internal injuries like this. My healing has limits and I cannot do everything in the world after all. I could easily take it out by putting my hand over her mouth and using my demon eye to channel my demon energy, and then vacuum all the demon energy with the soot, but as I said, that will only tear her apart from the inside out.
Check "The Protagnist's Sister Is Actually the Strongest"!