A "line crosser" can be a term used to describe someone who violates a boundary or rule. However, there are several synonyms that could also be used in place of this term. A "rebel" is someone who defies authority or conventional norms, while a "troublemaker" is someone who causes problems or stirs up conflict. A "boundary pusher" is someone who tests limits or challenges established rules, and a "line breaker" is someone who completely disregards boundaries altogether. Other synonyms include "provocateur," "disrupter," and "maverick." Each of these terms effectively conveys the idea of someone who is not afraid to cross a line or push boundaries, often to the discomfort or annoyance of others.