Kelly is a Marra. Like all Marra, she looks like a normal teenage girl during day time.
She has long, leg-length blond hair with a white hair clip inserted in her facial hairline. She wears a salmon-pink t-shirt with a white star in the middle. She wears a white long-sleeved shirt underneath the pink one with possibly a white bra underneath the white shirt. She wears light blue jeans with pink and white high tops.
She has all the powers of a Marra, including causing nightmares, and transporting herself in the form of a green mist.
Kelly is shown to be sassy and selfish, only caring about what she wants to do, such as when she watches a television show she likes, while Frida comments that it is terrible. She is also shown to be very mean, shown when she leaves Frida to get eaten by the Black Hound. She is also very forceful and takes advantage of Frida being her friend. On the other hand, she is shown to have a bit of a sensitive heart about certain things that she loves very much, such as her Fluff-Bun.
Since she lives in a regular house with her seemingly non-Marra mother, it is strongly suggested that Kelly wasn't always a Marra. How and when she became one however is unknown.
Kelly and Frida quickly got close. Kelly nicknamed Frida "Fri-Fri", the two created their own greeting, and hung out together to watch their favorite shows. Kelly, however, took advantage of Frida, since she was planning to eventually turn Frida into a Marra as well. To this end, she actively kept Frida from reconciling with her old friends. When in "Chapter 12" Hilda and David came to Frida's house to tell her about the upcoming Sparrow Scouts camp, Kelly also visited Frida, and the two decided to hang out together instead of with David and Hilda. Later, when Hilda came to Frida's house again, Kelly convinced Frida not to open the door, stating it was probably not for her anyway, and thus Frida never even knew that the visitor at the door was Hilda. It's possible that she also blocked Frida from being with the Sparrow Scouts, since when Frida began to get second thoughts about skipping Sparrow Scout camp, Kelly used her powers to enter Frida's room and reminded her of their plans for that night.
So, while the scouts were on camp, Kelly took Frida to a gathering of the Marra in the Huldrawood. There, Frida participated in the Marra's nighttime ritual; listen to other Marra telling stories about the nightmares they gave their victims, and then absorb the magic flames of the campfire. David and Hilda discoverd this while looking for Tontu, and tried to take her back, but Frida decided to stay with them. Kelly told them that she might make a good marra soon. Although Kelly tried to scare him, David stood up for Frida and called her out for joining the Marra.
But while he was talking, the Black Hound appeared, scaring Kelly and the other marra. It was now that Kelly showed her true colors, and pushed Frida towards the Black Hound for her to get eaten so Kelly herself could escape. Frida only survived thanks to Hilda and David.
Later, when Hilda, David, and Frida were collecting donations from people in the neighborhood, Frida crossed paths with Kelly once again when they approached Kelly's house. Kelly was surprised to see Frida still alive, and disgusted to see she was a Sparrow Scout again. Kelly's mother gave Frida a box of donations, including Kelly's old toy rabbit Fluff-Bun. Kelly panicked to see her beloved toy about to be given away, but was afraid to show her feelings towards it in front of Frida. Frida, seeing how much Kelly loved Fluff-bun, decided to let her keep it. She then told Kelly "It's never a good idea to throw away your real friends." As Frida and the others left, Kelly was stunned and looking at them, but then smiled and continues to snuggle her toy.
- Kelly's mother has red hair while she is a blonde, so its possible she got her hair color from her father.
- While the Marra appear in the novelization "Hilda and the Nowhere Space", Kelly is not among them. The novelization also omits the subplot of Kelly befriending Frida.