Even if we had the script, she would probably still seem like a last-minute afterthought. Trevorrow has never been good at writing compelling villains, and the plot outline that was posted doesn’t feature her much at all. She shows up for the final fight, dies, and that’s her entire screen time. She feels like Darth Maul 2.0.
Then that’s exactly what she should have been. Bring the entire story full arc referencing Episode 1’s Sith.
Have her wield the double blade, and have everyone question why is she a total Maul ripoff?
And then she reveals her identity, and why she hates Ben Solo so much: