Karen is one of the kids who built Frosty the Snowman in the Rankin/Bass special of the same name. She, along with Hocus Pocus the rabbit, accompanies Frosty on his journey to the North Pole. She was the only one of the kids who was named in the special.
Karen first appears when her class is supposed to be entertained by Professor Hinkle, but after his act proves to be a flop, she and her classmates go outside to build a snowman, whom Karen suggests they name "Frosty". After being brought to life by Hinkle's magic hat, Frosty finds himself in danger of melting due to the temperature rising, so Karen and her friends decide to bring him to the North Pole. On the way to the train station, Karen explains Frosty's situation to the Traffic Cop who stops them along the way. Unfortunately, since the kids have no money for tickets, Frosty instead chooses to stow away in the refrigeration car of a small freight train, with Karen and Hocus Pocus, Hinkle's rabbit, accompanying him, unaware that Hinkle is after them to get his hat back.
However, due to the cold temperature setting in the car to preserve the cargo, Karen starts catching a cold, despite her winter clothing, so Frosty decides to leave the train to find somewhere for Karen to warm up. Hocus rallies the forest animals to build a fire for Karen, but Hinkle then ambushes Karen and puts out her campfire, forcing her and Frosty to belly-slide down a hill. They find a greenhouse growing Christmas poinsettias, where Frosty carries Karen inside so she can warm up, even though it could cause him to melt. Unfortunately, Hinkle catches up and slams the door shut, trapping them inside.
By the time Hocus brings Santa Claus to the greenhouse to help Frosty by following his tracks, they are too late; Frosty has melted, and Karen is heartbroken over her friend's demise. However, Santa consoles her and reveals to Karen that Frosty, being made from Christmas snow, just needs a cold, December wind to be revived, as he then demonstrates before Karen's eyes. When Hinkle again demands his hat back, Hocus seeks Karen for protection while Santa deals with the inept magician. After offering a fair compromise to Hinkle, Santa restores Frosty to life, much to Karen's joy. However, Santa has to take Frosty back to the North Pole with him, but first brings Karen back to her home. While Karen is sad to part ways with Frosty, she is assured that Frosty would return next Christmas to lead Karen and her classmates in another parade.
Sometime before the special first aired (possibly a last-minute change made during post-production), Karen's dialogue (originally provided by June Foray) was re-dubbed by another actress, possibly in order to fit her characteristics. However, Rankin/Bass forgot to change the credits, so for many years and decades her replacement actress remained unknown. It wasn't until December 2017 that sources started listing Suzanne Davidson (best known for her lead role as Susie in the 1973 TV remake of Miracle on 34th Street as well as the original actress to play Betsy on As the World Turns) as the replacement voice for Karen.
Despite her prominent role in the original special, Karen (along with the other kids from the original special) was oddly absent from the sequel, Frosty's Winter Wonderland.