Who or What is Roo???

So, I don't know a lot about Hazbin outside of the Pilot, and the show (I'm catching up with Helluva) but I'm just confused on what or who the hell Roo is??? People mentioned her all the time, talk about her being this big bad, or the root of all evil and I'm like, is she in the show??? Did I miss something??? So could someone please explain this to me?