Sue Walsh (Sianoa Smit-McPhee) is the main villainess from Criminal Minds episode 9.15, "Gabby" (airdate February 26, 2014).
As a young girl, Sue was sent to live her cousin, Kate Hoffer after her parents died from unknown causes; however, Kate's father molested Sue, leading Sue to become jealous towards Kate, who was never sexually abused by her dad and had no idea of her father's actions against Sue.
As an adult, Sue decided to get revenge on Kate by having her four-year-old daughter, Gabby abducted when Kate leaves her in Sue's care while she is away on vacation. Sue recruited her boyfriend, Ian Little and Kate's ex-husband, Doug Hoffer to help her out, but when Doug refused to leave Sue's house without seeing Gabby and he roughly shoves Sue, Sue murders him with a glass candlestick, and Ian disposed of Doug's body.
After that, she changes her plan by contacting an online forum about disrupted adoptions and contacts a woman named Nicole Jones, who agreed to take Gabby. Sue takes a sleeping Gabby to a convenience store where Nicole abducts Gabby from the car. When Sue returns, she feigns horror and screams for help. When the BAU is called in to assist in the investigation of Gabby's abduction, Sue is believed to be innocent, but they become suspicious after Doug's body is discovered and security footage from the convenience store shows that Sue didn't park her car near a light (which is something that a protective mother would do).
When Sue is being interrogated by Agent Hotchner, she attacks him in a fit of rage when he accuses her of being responsible for Gabby's abduction. Afterwards, the evil Sue continues to be uncooperative with the BAU, while making vague statements hinting towards how she was abused by her uncle. Later on, Kate offers to help the BAU interrogate Sue and when she does, Sue tells Kate that Gabby is gone and she also taunts her by implying that she is pregnant and boasting that she would never allow for what happened to Gabby happen to her child. After that, Sue isn't seen for the rest of the episode, but was likely incarcerated for her actions off-screen.