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, Karen and her classmates go outside to build a snowman. Once they do, Karen suggests they name him "Frosty". It is then that thanks to Hinkle's rabbit assistant, Hocus Pocus, that they acquire Hinkle's magic hat, which brings Frosty to life. After Hinkle takes his hat back and Hocus returns it to the kids to reanimate Frosty, Frosty realizes he needs to find colder weather as with the temperature rising, he is in danger of melting. The kids suggest the North Pole, and escort Frosty as they march through town to the train station, with Karen explaining 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 accompanying him to the North Pole, 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 to catch a cold, and Frosty decides to leave the train to find somewhere for Karen to warm up, which works when the train pulls into a siding to allow a passenger train to pass, allowing them to unknowingly escape Hinkle again. 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 to where they find a greenhouse growing Christmas poinsettias, where Frosty takes Karen inside so she can warm up. Unfortunately, Hinkle catches up and closes the door, trapping Frosty and Karen inside, gloating about how he will soon have his magic hat back.
By the time Hocus brings Santa Claus to the greenhouse to help Frosty, they are too late. Frosty has melted, and Karen is heartbroken over her friend's demise. However, Santa reveals to Karen that Frosty, being made from Christmas snow, just needs a cold, December wind to be revived. Before Hocus can add the magic hat back on, Hinkle again demands his hat back as Hocus seeks Karen for protection as Santa confronts Hinkle and threatens to take him off the Nice List for life if he laid so much as another finger on the hat's brim. However, Santa offers Hinkle a compromise: If he returns home and writes his apologies repeatedly several hundred times over, Santa would reconsider removing Hinkle from the Nice List and still give him a gift on Christmas morning. Moved to repentance at Santa's showing of mercy, Hinkle leaves to do just that, and 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 where Karen hated to part with Frosty, but was assured that Frosty would return next Christmas to lead Karen and her classmates in another parade. Karen waves goodbye as Frosty and Santa return to the North Pole, and come next year, Karen parades proudly behind Frosty through the streets with her classmates until they reach Santa's sleigh and Frosty takes his leave to return north. Karen and her classmates chase down Santa's sleigh until it gets airborne, then wave goodbye together as Frosty assures them they will see him again on Christmas Day.
Despite her prominent role in the original special, Karen was oddly absent from the sequel, Frosty's Winter Wonderland.