I unironically think Piano Lady is part of the Elite 8. Her design feels unique enough to warrant her being someone very important later on, (I know design isn't much to go off of here, so I could be totally wrong lol). But I don't know, I have this feeling she's some sort of wild card, and the fact that she's one of the first unique NPCs we can interact with on the SS Oceana gives me that feeling. Although she doesn't seem to be someone very powerful when we first meet her, I think this can be explained by her just being modest? Maybe from her perspective its a bit rude to bring up your status to new trainers about to begin their journey, and I respect that kind of attitude. That said, learning about Celine in V13, and the kind of people she chose as Elite members It kind of makes sense that she'd be drawn to someone like her. All of the Elite members we've meet so far seem to be kind hearted people (Tesla and Dylan are good examples I think). If I'm remembering correctly and someone correct me If I'm wrong here, but there is a small scene at the end of the Goomink quest with Celine. She's talking to Goomelda about hiding the light stone at the castle and entrusting her to keep it secret. Her quote at the end was "After all, love will prevail" I think? It seems to me like Celine was the kind of person to see more potential in kind-hearted people, rather than outright powerful ones. So why not someone like Piano Lady?
I probably look pretty dumb here writing a whole paragraph about this, so I wont say I told you guys so If somehow I end up being right haha. But Jan sure loves to surprise us with this game so who knows.