What we have here is a failure to use proper noun-verb agreement… Context alone doesn’t make it clear whether the writer meant to be talking about one girl or more than one.
“And when you’re the girl who’s known for being invisible, there’s nothing quite as scary as being seen.”
“And when you’re the girls who’re known for being invisible, there’s nothing quite as scary as being seen.”
I have to assume invisible is meant to be taken more-or-less literally, because invisible meaning unnoticed doesn’t fit with known for. (People who are unnoticed to the point of being effectively invisible don’t have a reputation for anything, do they? “Hey, have you heard about that kid no one ever notices or even thinks about?” *shakes head*)