Gillian was the best friend of Samantha Hodges (the main protagonist) and Nate Squires; having known the latter since kindergarten. The three of them were up for an important scholarship, along with Jordan Reynolds, Rudy Kampion, and Brittany Gomez. However, Gillian was also obsessively in love with Nate, and in a twisted plan to attempt to win his affections, she devised a murderous scheme to make sure that he received the scholarship.
Gillian's machination began with killing Jordan, doing so by lacing Angela Dodds' lip gloss with peanut oil--knowing that Jordan had a peanut allergy. It was at the beginning of the film, during rehearsal of the school's play, that Jordan succumbed to the effects after he kissed Angela. Gillian's next target was Brittany, who she went after at Rudy's party at his house, which is where she also made an attempt to flirt with Nate, only to realize that Nate had an eye for Samantha. The villainess publicized a photo of Jordan (who Brittany was dating) making out with Lance Hobart, getting the photo from Nate's computer, and then originally planned to drug her and set her up to be accused of driving drunk. However, the evil Gillian instead decided to kill Brittany, using a knife belonging to Rudy's father to commit the murder.
Angela was accused of Jordan's death due to Gillian's actions, which later included setting up Rudy as a school drug dealer by planting drugs and the knife in his locker. Samantha even believed that Nate was the one behind the leaking of the photo of Jordan and Lance, and informed her suspicions to Gillian. Gillian stated that Nate saved the photo, but revealed that she was the one who publicized it, and it was there that she unleasned a knife and pointed it at Samantha, completing her reveal as the killer. Then she revealed her affections for Nate, stating that it was the reason why she transferred to the high school six months prior, while also lashing out at Samantha for (in Gillian's mind) stealing Nate from her. She stated that Nate was her best friend and soulmate, and that she did everything for him, while also saying that she killed Brittany because she was angered by her complaining and that she was going to kill her too.
At that moment, Samantha callled out Nate, who revealed that he had been recording Gillian's confession. Gillian then decided to eliminate both of them, only for Samantha to throw powdered lye at Gillian to distract her. Nate later clocked Gillian with his camera, which only prompted Gillian to pursue Nate and attempt to kill him. The twisted villainess professed her love for Nate, and it was at that moment that Samantha struck Gillian with a wooden plank, killing her.
- "Best friends do extraordinary things for each other." (Gillian's direct statement revealing her affections for Nate and her murderous motive)
- "My idea was to just get Brittany wasted and then make an anonymous call to police that she was driving drunk. But you know what? When that entitled, pretentious whore just sat there whining on the window sill, after all that she's been given, she just had to go." (Gillian confessing to killing Brittany)