As for it being a secret, usually you get "temporary" bigs (heart sis for example) for the new member period. Shortly before initiation, there's usually a big sis revealing, where, through creative and fun ways, you discover who your big sis is. Sometimes you get some input, sometimes the members pick, that depends on the chapter.
But either way, it's someone who's a sort of mentor for you. And makes sure you know about all the er, unofficial events that may be taking place on Friday night
