[@ZxyxZemon] It's similar to the gamma radiation one but instead of one person mutating into something...the character is being possesed by a demon, giving said character a double personality. While in a human form, they're just faster and stronger than the average Joe but while being in demon form, the demon takes full control becoming a killing machine. Faster, stronger, can hide in shadows and idk, need to think more about abilities in the demon form.

The demon and the human would have a good relationship and could communicate between each other at will. My answer to the question : "What if instead of a guardian angel, you had a guardian demon?"