Gretchen Ross is a key character in Donnie Darko, and Donnie's romantic interest during the events of October 1988.
She moved to Middlesex, after an incident in which her step-father tried to kill her mother. In the tangent universe, Gretchen meets Donnie after attending his school, and the two eventually become romantically involved.
Towards the end of the film, she is killed on Halloween night when Frank runs her over.
As the events throughout most of the movie took place within an alternate universe, everything that "happened", including her death, did not actually occur.
Little is known of Gretchen's background, other than her and her mother fled their original lives after Gretchen's step father tried to kill her mother. After fleeing, Gretchen and her mother moved to Middlesex. She begins attending the same school as Donnie.
Gretchen meets Donnie after he floods the school. They bond and Donnie asks her out. After several dates, Donnie tries to kiss Gretchen, but she tells him that she wants to wait for the right moment. One day, after the school bullies mock her about the incident with her step-father during class, she runs out of the school in tears. Donnie follows after her, trying to console her, she stops, turns around, and kisses him then and there.
Donnie and Gretchen's romantic bond grows, and they continue seeing each other throughout the days ahead.
On Halloween night, Donnie's sister throws a Halloween party. Donnie and Gretchen grow even closer when Gretchen eventually shows up at the doorstep, informing Donnie that her mother has vanished and that she is terrified that her step father has found them. Donnie and Gretchen go to a room upstairs alone, where he comforts her. Shortly after, Donnie and Gretchen end up having sex. Later, when Donnie realizes that Frank's prophesized end of the world is mere hours away, he leaves the party to go find Roberta Sparrow, who he believes has answers for what is unfolding. Gretchen and his two friends accompany him, and they eventually arrive at Roberta's house to find the two school bullies there. Donnie's friends flee, while the bullies assault and threaten Donnie and Gretchen. After a struggle, the bullies have them pinned on the ground, threatening them with knives on the road outside of Roberta's house. A speeding car emerges from down the road, the bullies flee the moment they see it. As Donnie and Gretchen lay on the ground recovering, the car approaches and Roberta Sparrow emerges, wandering around the street near them. In a matter of seconds, the driver of the car notices Roberta last minute and swerves to avoid hitting her, turning the car towards Gretchen, who is still laying on the road. She looks at Donnie and screams his name before the car runs her over, killing her. The driver of the car and it's passenger emerge. The driver is revealed to be Frank, still in his nightmarish rabbit suit. However, this Frank appears to be a normal, human version of himself, rather than his paranormal, doom predicting variant, and was simply going out to buy liquor for a party, wearing the rabbit outfit as a Halloween costume. Frank and the passenger of the car stand beside it and look in horror at what they've done. A shocked Frank looks at Donnie and says "What were you guys doing in the middle of the road, huh? What where you thinking?!". Before Donnie impulsively pulls out his father's pistol and shoots Frank dead to avenge Gretchen's death. Frank falls to the ground, dead from a gunshot wound to the eye. The passenger of the car runs away, leaving Donnie to stand there as Roberta Sparrow continues wandering around nearby. With the police soon to be informed of his actions, he flees the scene, carrying her body back to his house. He places the deceased Gretchen in the passenger seat of a car, before driving into the outskirts of Middlesex to save the universe.
After the collapse of the tangent universe, in which the majority of the film's events occured, Donnie is killed by a jet engine which falls through his house into his room. The following morning, while Gretchen is riding her bike, she sees Donnie's body being carried out on a stretcher. Since the events of the movie took place within the tangent universe, including their meeting and romantic connection, she does not remember anything about him, and simply reacts with slight sadness at his death.