The Council Leader's Aide
Sisera used to be the Council Leader's aide. She was best known around Thule for her ability to open portals, which made her invaluable within the Grand Acropolis. Generally, she appeared in the Council Leader's stead, often alongside Navarriel.
She also was referred to as "The Ancient Thorn" due to her curious ability to always be at the right place and time to learn people's darkest secrets and exploit them.
Prior to the Fall of the Beyond, she was responsible for keeping the Princes happy. She was also responsible for Nox's initial ailment and the Watcher's seals that still plague him to this day. This is precisely why he murdered her, much to Mayuri's resistance.