She spends 99% of her scenes looking like she's sucking on a lemon. It's weird.
As a character, I remember thinking there were sparks of something interesting here and there (when she killed the last member of the triad, who essentially raised/trained her) but it never amounted to anything. Her whole arc was a mess. Billie could have worked as a character had they taken the time to work her into the series better, and not wrote her into the ground in service of turning her against the sisters. I think Kaley did what she could, and we know she has the range now, but maybe if Billie was cast with a more dramatic leaning actress it could have helped somewhat.
The Halliwells themselves were also awful in that final season. Phoebe was a one-dimensional moron, Paige was a bumbling idiot (partially a product of Rose spluttering her way to the finish line since she was over it all, for reasons we all understand now), and Piper had morphed from prickly but likeable into a total whinge-bag by the time we hit late season 7. Honestly, a good chunk of the reboot eclipses that final season of the original.
