In the book, Greta is described as having shoulder-length blonde hair, wears nice clothes and is described as to having her hair permed every month. In the film adaptation, she has dirty blonde hair pulled back into a ponytail.
She first appears in Ben Hanscom’s flashback on the last day of 5th grade, Mrs. Douglas’ class, Ben describing her and Sally Mueller as a pair of stuck-up snobs due to their rich families, nice clothes and permed hair. Her name is later called out when Mrs. Douglas is handing out report cards.
Later on in the Summer, Richie comes across Beverly Marsh and mentions how pretty she is and soon says that Greta and Sally loathe Beverly for being extremely poor but still prettier than them. Beverly and Richie begin to make fun of Marcia Fadden, Sally Mueller and Greta Bowie saying that they “pee rose water”, which is their way of saying they are unlikable rich kids.
On the night of Beverly’s encounter with IT, Beverly remembers Greta and Sally talking about sex-acts.
Later on in 1985, Eddie Kaspbrak walks down West Broadway which was the wealthy parts of Derry and where Greta grew up. As Eddie walked down the street of Victorian houses, he could remember one day during his childhood that he came across Greta’s house which is described as a large Victorian house that’s green-shingled with cone-shaped turrets on each side that Eddie says look like "squatty duncecaps". Eddie saw her and her family playing croquet and drinking lemonade in their backyard during one summer day. Eddie described Greta as being “blonde and very pretty”, she was also said to be wearing a cool blue culotte dress.
Greta is later seen as a manifestation of IT, informing a grown-up Eddie that she died in a car accident. IT tells Eddie that she was 18 and under the influence of alcohol and several pills while driving. Greta, Patrick Hockstetter and a boy named Tony Tracker were chasing Eddie, all wearing ruined baseball trikots and appearing as decayed corpses. Greta had half of her face missing and was covered in dried, crusty blood.
Greta appears in the 1990 miniseries but is renamed Loni. She briefly appears in the first part with Sally Mueller, walking through Ben and Beverly, acting snobby and stuck up, while Sally says that some people have no class, while Loni replies that some people have janitors for parents, before giving Ben and Beverly a snobby look. She is not seen again.
It: Chapter One (2017) & Chapter Two (2019)
At the beginning of the film, Greta is shown to be a bully when she goes into the girls' bathroom and begins harassing Beverly Marsh by calling her a "slut" and a "little shit". Greta also wrote LOSER on Eddie's arm cast and he later covers the S with a V in red marker, so it then spells LOVER. She is also the one to tell Eddie his medication is a placebo rather than Mr. Keene himself.
In the sequel, she is seen to still be alive, still living in Derry and working with her father, however her attitude hasn’t changed.