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. Karen suggests they name the snowman "Frosty" after rejecting several other names. After Hinkle's magic hat brings Frosty to life, and Frosty realizes that he is in danger of melting due to the temperature rising, 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, so Frosty decides to leave the train to find somewhere for Karen to warm up. But after Hocus rallies the forest animals to build Karen a fire, Hinkle 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 takes Karen inside so she can warm up, even though it might cause him to melt a little. Unfortunately, Hinkle catches up and closes the door, trapping Frosty and Karen 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. After Santa offers 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 takes Karen back 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.

During the third airing of the special, Karen's dialogue (originally provided by June Foray) was redubbed 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 second actress remained unknown. However, in December 2017, sources started listing Suzanne Davidson as the replacement voice for Karen. Davidson is 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.

Despite her prominent role in the original special, Karen was oddly absent from the sequel, Frosty's Winter Wonderland (as well as all the other kids from the original special).