Catherine Stewart a.k.a. the Cat
Catherine Stewart, almost always referred to as the Cat (note, the is not capitalised), is a criminal who is responsible for heinous crimes against innocents, which are left undisclosed throughout the books. Although what happened on Narrogin was probably it.
Commonwealth Saga
In this period of time she is described as being relatively short, having short dark hair, and being very devoted to yoga, often performing it during rather intense circumstances. She was sentenced to a very long period of suspension (5,000 years), but was revived in order to fight the Prime alien MorningLightMountain's attempted genocide against humanity, joining a group of similar criminals who adopted the moniker Cat’s Claws in her honour.
The Void Trilogy
After the events of the Starflyer War she remained on Far Away, where she transformed the then purposeless Guardians of Selfhood into the Knights Guardian, as described in The Dreaming Void when Oscar Monroe first meets Tomansio. She is first encountered when Troblum is introduced to her by Marius. Troblum was working for the Accelerator faction, believing it had merit. But upon meeting the Cat, he decided that it must be evil.