My bad ... thought it was through LoC ... even still, TFoH give us plenty of insight into Graendal,

She's a master of compulsion

She's one of the more "Open" of the Forsaken, meaning not one to shy from things she needs. She's smart, but unlike Moghedien she's not afraid to act openly.

She's willing to kill other FS, as we see when she talks openly about it in the meeting with Sammael, Rahvin and Lanfear... and later almost attacks Lanfear.

We know Asmodean knows her location

She's spying on the other FS in TAR according to Birgitte

I also don't think it's a coincidence that she's introduced in the same book as Asmo reveals his knowledge of her location and is then later killed.

But as you say, we'll have to RAFO!