Thalia is a Branched who was imprisoned long ago as Somerset House by the ancestors of the Solstice family. They are rescued by Guilford Gain and her accomplices Laris, Leah, and Laurice, all members of Millennium Break, at the end of "Upstairs & Downstairs Pt. 2". When they remove a scale model of the house hidden in the basement, Thalia is freed.
Thalia's rescue causes the exterior hedge maze to bloom with tulips and Somerset House to collapse in on itself. During their flight from the house, Thalia takes a humanoid form made of stems and rose flowers.