Shadowheart feels a lot more like the Sebille of this game, to be honest. Right down to the fact that if you're playing the wrong race (Gith or Lizard respectively) there is a high chance you'll end up killing her upon your first meeting.

They're both cagey about their pasts and in service to some dark power they don't want to talk about. They both even have
a loss of memories of who they are supposed to be
as a plot point. And they're both elven and snarky.

Astarion has little in common with Sebille aside from general rogue traits.