Right now, she's in Immersive Theater, the 2nd and 3rd stage. If you're a new player, you probably won't be able to clear it, or at least, not all of it (enemies go from lvl 50 to 80+, and you need certain type heroes). Another way to get her is to simply wait, she and her weapons will replace rhytm in the weekly boss. You need around twenty to thirty character secret letters for a guarantee, and around ten to fifteen for a weapon, depending on how many unique parts it has. Hope you get her!