A guy who can steal people's identity and power - what is this anime?
It could be DNA² you are talking about



The girl in question seems to be Tomoko and as soon as the villain Ryuji touches her, he changes to her I guess. I don't remember if the last line of sucking others life force. But the description brought back the scene from DNA². Check out episode 10 of the series and tell if it is the one you want. Hope this helps :D