Shadowheart might not be very original, but neither are Amiri and Jaethal. It doesn't mean she is badly written. I'm surprised you didn't mention Octavia, a companion who was actually kind original for an RPG (but that doesn't mean she was well written)

Larian's Biggest Oversight, what to do about it, and My personal review of BG3 EA
"74.85% of you stood with the Tieflings, and 25.15% of you sided with Minthara. Good outweighs evil, it seems."