Man, Medusa is ****.
MANGA SPOILERS
She manipulates her daughter (son? Damn it Crona), she manages to fight the best technician and Death scythe evenly, she only loses the fight because she, in a sense, WINS. Even then, she doesn't die, she manages to split her soul into pieces (I think), pull them together in one of her snakes, makes the snake ENTER A LITTLE GIRL, THUS TAKING HER OVER, thus reviving herself. She then goes on to make Stein go completely insane, turn poor Crona into a mole, kill an especially gifted soul perciever, blame that murder on Stein, and then makes Shibusen give her the permission to lead an assault on her rival-like sister. And taht's with Shinigami-sama's (albeit reluctant) approval.That's ****, awesome villany right there. Oh, and her scary face makes her look AWESOME.