Rengoku used to be a demon slayer until his fight with akaza.. Unlike in the book though he actually says yes to akaza when he asks if he wants to turn rengoku into a demon. After turning into a demon his mind currupted but one thing that didn't change is his caring look, even while he may be evil. Rengoku is now also looking for the blue spider lily just like the rest of the demons.. Although their is a way to calm him..