The princess Dirk, Charlie's adopted brother, married in the beginning of Tress is a member of a Selish Secret Society with a plan to kill the Sorceress.

What evidence is there for this you ask?

1. The name of her kingdom is Dormancy, proving is connected to Sel
2. She is described as exceptionally beautiful and her hair shimmers, which clearly indicates Investiture based beautification
3. She appeared on Diggen's point shortly before Tress started the journey that drove the Sorceress off-World. Which means her Selish Secret Society has an ability similar to Hoids ability that pulls him to where he "needs" to be.
4. She married a War Hero, who is clearly the strongest Sorceress Slayer on Lumar