One who constantly steps over "the line" whatever that line may be. This has the effect of forcing you as an observer to set him or her straight with physical force or the threat of physical force.
RICK JAMES "Charlie Murphy!!! (punch to the head, leaving a mark)
Then Charlie Murphy proceeds to kick Rick James' ass because he's gotta check that shit so it don't happen again, forcing him to stop being a habitual line stepper.
A stepper is a person known to carry guns and is always ready to fight, typically known to not fear jail time. Usually quiet people not known to start anything but will nothesitate to engage in any kind of fighting.
A total bitch who comes along hogging your best friend and is really annoying. Your friend likes them but you despise of them. They act like they are all innocent but you know that they are evil deep down.