Casey (Niki Koss) was the main villainess of the 2015 Lifetime film My Stepdaughter.
Casey was the daughter of Robert and Nancy, and the opening of the movie showed Casey as a child, tearfully watching as her parents fought, with the psychotic Nancy accusing Robert of cheating on her after following him and then seeing him with another woman. Robert revealed the woman has his divorce lawyer before going to comfort Casey. Nancy responded to the news by smashing a vase and trying to kill herself with a shard before Robert stopped her. As Robert called 911, Casey picked up a shard of glass and cut her arm with it.
It was later revealed that Nancy gained custody of Casey for five years, with heavy implications being that Nancy abused her. Casey spent some time in a mental institution before being sent to live with Robert, who was now married to Jill and was the stepfather to her daughter Lydia. Jill attempted to forge a bond with the troubled teen, but Casey adamantly refused.
In the beginning of the movie, Casey was having dinner with Jill, Lydia, and Jill's mother, snapping at her step-grandmother for a comment regarding her Gothic appearance. At school, Casey taunted her ex-boyfriend Victor before flirting with shy new student Brad. Later, Casey enlisted Brad's help in pranking Victor, having him plant a camera in Victor's car and record him as Casey callously rejected him, later uploading the video to the Internet. That night, however, Casey's actions turned darker, as she (offscreen) went to the nursing home Jill's mother was staying at and killed her by injecting her with a poison. She also replaced the elderly woman's will with a typed up one leaving Jill's irresponsible sister Skyla with everything.
Rather than going to the funeral service, Casey left the house with Brad to go to an underground rave, much to her father's anger. While at the party, Casey showed several partygoers the video of Victor, causing him to confront her. The next morning, Victor was found dead by his friends and Casey smirked at Brad as people crowded around his body, silently revealing that she'd killed him. Jill began to first uncover Casey's evil deeds when she found a necklace similar to one she gave Lydia in her mother's nursing home bedroom.
As a result of her disappearing at the rave, Casey was grounded, but was permitted to babysit Lydia while Robert and Jill went out for a picnic. At the house, Casey and Lydia did a puzzle while Casey subtly tried to turn Lydia about her mother. After learning about Victor's death, Jill got into a fight with Casey when she told her that she'd killed Skyla, who was visiting. When Jill went into Casey's room to talk to her, she caught Casey opening the safe she kept under her bed. She tried to see what was in it, but Casey stopped her and Jill left when Skyla came in. While Casey was out, Jill unlocked Casey's safe and found the poison she used to kill her mother and Victor. Casey and Robert caught her, with Casey having a meltdown and forcing them both to leave her room. Having found out about the will, Casey had wine with Skyla, getting her drunk and turning her against Jill, causing a fight between the two and causing Jill to kick Skyla out.
After Jill found out about the poison and realized how dangerous Casey was, she went to pick up Lydia from school and found out that Casey had picked her up with Brad, kidnapping the young girl and telling Brad that they were saving Lydia from an abusive home. When she got back to the house, Casey Skyped Jill on her phone, showing her a tied up and gagged Lydia. Jill called Robert to tell him, and he deduced that Casey had taken Lydia to a forest they went to when she was a child. When Lydia asked Casey and Brad to get her water bottle from the car, she freed herself and tried to run away, but was eventually caught and tied to a tree. Casey then prepared to set her stepsister on fire, fatally stabbing Brad with a knife when he tried to stop her from dousing the tree in gasoline. Jill arrived and tried to convince Casey to let Lydia go, saying that she loved her and wanted to help her heal. Casey feigned understanding before trying to stab Jill, who managed to push Casey down and use the knife to free Lydia. As Robert arrived, Casey charged at Jill, causing her to impale herself on the knife, killing her.