She was portrayed by India Eisley, who also portrayed Airam in Look Away.
As a baby, Heather was left at a hospital by her birth mother. She then went into the care of foster parents Gale and Jake Lambert, where she was sexually abused by Jake. When she turns 18, she runs away from them and searches for Linda Kessler, the woman she believes to be her birth mother. After finding out she had started a new family consisting of husband Mark and young daughter Chloe, a scorned Heather plots revenge.
After the Kessler's nanny gets dementia and accidentally hurts Chloe, Linda looks into hiring a new nanny. Heather then applies for the job after running into Mark at the construction site where she stopped Chloe was playing with a nail gun. Linda calls her references and they speak highly of her, but it is implied that the "references" were actually Heather disguising her voice over the phone.
After getting the job, Heather grows close to Chloe, even subtly threatening a kid who is bullying her. She also begins to clean the house while Linda and Mark are out and convinces Chloe to do a monologue for an upcoming talent show instead of the violin like Linda wanted, unnerving her. She confronts Heather about this and she feigns understanding. Linda is also starting to catch on to Heather's lies when she finds out that a friend of hers who went to the high school Heather claimed to have attended does not remember her. Meanwhile, after having a confrontation with Chloe's old babysitter Mrs. Highsmith, Heather sneaks into her house and kills her, staging it to look like suicide.
Heather also begins trying to seduce Mark, and when she finds out about the nanny cams Linda installed due to her suspicions, strips for them for Mark. On the day of Chloe's talent show, Heather fakes a panicked call to the police, claiming Linda had kidnapped a child. The police stop Linda on her way to show, causing her to miss it and earn Chloe's ire. Linda is angry when she learns Chloe did a monologue instead of the violin, and also confronts Heather about her lies despite Mark's insistence she stop. When she threatens to call her parents, Heather begs her not to and reveals how her parents were abusive, leading to her running away. Linda still does not believe her, which causes her to fight with Mark, who brings up how Linda made mistakes when she was young.
But after Heather gives Mark a spiked drink making him pass out and finds the nanny cam footage of her stripping, Mark agrees with Linda that they need to fire her. Soon after that, Heather puts a video of Heather and Mark having sex while Mark was passed out in a work presentation of Linda's, causing her to confront Mark. Chloe, angry that her mother fired Heather, runs up to her room to cry, where Heather is hiding under her bed. Heather comforts Chloe before kidnapping her, leaving a video message on Linda's laptop that if she wants to see Chloe again, she will meet her at Mark's construction site.
Linda goes there and finds a secret room in the attic, where Heather has been living. Heather is there, holding a nail gun to Chloe's head. It is there that Heather forces Linda to reveal to Chloe how she got pregnant as a teenager and left her baby at the hospital, claiming to have been that baby. But Heather reveals that she is not, having gotten a necklace from her daughter's adoptive mother after she died in a car accident. Heather refuses to believe this, causing them to fight over the nail gun. As Heather knocks Linda to the bed, Mark shows up and feigns acceptance to Heather's plan to kill Linda. Heather instead shoots Mark with the nail gun several times and attempts to kill Chloe, but Linda attacks her and pushes her through a window, seemingly killing her. But the next time we see the ground where Heather fell, she is gone.
The final scene shows Heather on a train, having survived the fall, now with blonde hair and holding a baby. When an elderly woman asks what the name is, Heather says he is named after his father Mark, revealing that she was impregnated by Mark. Nevertheless, despite her plan being foiled, Linda and Mark, along with their daughter Chloe, move to a new home to enjoy a much more happier life.
- Her age is unknown, but she could possibly be in her early 20s.
- At the end of the film, it is unknown if her hair was actually blonde; in fact, it could actually be a wig that she's wearing as a disguise so she can hide her true colors following her defeat to Linda.