Gabby

Hotchner: It's often said, "There is no footprint too small to leave an imprint on this world."

A woman was raped by her uncle when she was a child, and kidnaps her cousin's daughter as revenge on her cousin for being spared.

Tropes in this episode

Batman Gambit: Hotch calls Sue Walsh pathetic to get her to reveal her true sociopathic self.
Clashing Cousins: Sue Walsh and Kate Hoffer appear to have a pretty good relationship. Then its revealed that Sue secretly despises Kate because the latter's father sexually abused Sue as a child and Sue decided to take revenge by arranging for Kate's daughter Gabby to be kidnapped by pedophiles.
Dead All Along: The missing girl's father is a suspect, right up until his body is found and it turns out he was dead before the abduction.
Revenge by Proxy: Sue wants revenge on Kate because Kate's father raped Sue when she was a child and living with them. She takes revenge by kidnapping Gabby, Kate's four-year-old daughter.

Jareau: "A mother's arms are made of tenderness and children sleep soundly in them." Victor Hugo.