She was Jigsaw's protege and helped him with his traps and games. After Jigsaw's death, she takes on his mantle and continues his legacy of "teaching" people life lessons through torture.
However, it's important to note that the franchise continues to explore the motives and identities of other killers who operate under the Jigsaw "brand," further blurring the line between who is the true killer and who is simply continuing the legacy.