Katie Cavanaugh

Katie Cavanaugh is the secondary villainess from "Totem," episode 12.20 of Law & Order: SVU.

She was played by Agatha Nowicki.

Backstory
Katie Cavanaugh is the younger daughter of Elaine Frye Cavanaugh and the half-sister of piano teacher June Frye (Elaine's daughter from her first marriage). Both half-siblings were victims of Elaine's sexual abuse; though in the case of Katie, it was ongoing into adulthood, as she and Elaine not only lived together, they shared a bedroom. The abuse affected both women; Katie more so than June, as Katie became traumatized throughout her childhood and into adulthood.

Events
As a result of the abuse, Katie followed in her mother's footsteps and became a molester herself, and in the episode, she approached Marnie (one of June's students) and attempted to undress her, only for Marnie to refuse. In a rage, Katie took June's sleeping pills and drugged Marnie, and later used a wooden spoon to sodomize her victim--after which, Katie suffocated Marnie to death with a pillow. After seeing what her sister had done, June took Marnie's body and placed it in a duffel bag, with a doll inside; while also taking the rap to protect Katie.

Katie's reveal (among many other revelations) came when Benson and Stabler confronted Elaine (after learning that she was alive) at her home, where they first saw Katie. Katie became nervous while devising an alibi for Elaine, and it was at that moment that she ran into the bedroom and hid in the closet. She is later taken in by the pair, and it was at Riker's that Katie embraced June, who tearfully apologized and blamed herself for not protecting her. Later on, Katie confessed everything to Stabler, while June watched. Olivia informed June that Katie will be sent to a psychiatric facility, and the episode ended with Katie telling June that she loves her and will always be her little sister.