a person who has a bad influence on you - it is like a cartoon devil sitting on your shoulder and making you commit sin, crime etc. - from Mephistopheles - the demon in the Faust legend
Frank: What happened to Jeremy? He used to be such a good student and all.
Susan: Well, he started hanging out with this new boy, Jason, a real Mephistofella, that dragged him into cigarettes, alcohol and drugs.