What does SN mean from a girl?

Could you clarify the context in which "SN" is being used by the girl? It could potentially have a variety of meanings depending on the situation. However, in general, if "SN" is being used by a girl in a social or messaging context, it could potentially stand for "screen name," which is a username or handle used online. Alternatively, it could be an abbreviation for something specific to a particular group or community that the girl is a part of. Without more context, it's difficult to say for sure what "SN" means in this case.