Karen specialises in Dark-type Pokémon, and is initially part of the newly formed Team Rocket alongside Will. However, once Team Rocket's plans are foiled, they are approached by Bruno and Koga, who invite them to join the new Elite Four.
Karen has a cruel personality, as shown when she willingly sided with the Masked Man, but under the guise that they were kidnapped alongside the others. She is shown to despise Green for her ornithophobia (fear of birds).
Karen is exceptional in her control over Pokémon, as shown when she was able to control the Legendary PokémonHo-Oh in the final battle at Ilex Forest Shrine with relative ease. Both Karen and Will were considered to be geniuses, and that no-one could best them when it came to Pokémon battling.
Karen is identical in appearance to her game counterpart. When she becomes a member of the Johto Elite Four, Karen's appearance is rectified to match her revamped game outfit. She also wears one of the masks shared by all of the Masked Man's subordinates.
Karen was initially recruited willingly alongside Will into the service of the Masked Man. When Ho-Oh snatches Green and takes her to the Bell Tower, Karen and the other associates of the Masked Man appear before her. Later, when Green and Silver attempt to escape from the Mask of Ice's base, Karen and the other associates intercept their movements in the air duct. Karen attacks with her Houndoom, as Silver sprawls from the combined attacks. Green severs a cooling vent, as the billowing smoke obscures their movements from Karen and the others. When Karen and the others pursue them, they find themselves faced with the gradually expanding body of Green's Jigglypuff. Later, Karen becomes one of the Team Rocket executives of the Johto region under the Masked Man's plan.
Gold, Silver and Crystal Chapter
Karen appears at the Ilex Forest Shrine to confront Green alongside Will. As Green prepares to attack Karen with her Snubbull, she uses her Umbreon to incapacitate it. Karen reveals that the two of them have been instructed to guard this place from any intruders, and that the Mask of Ice will gain the ability to manipulate time. When Karen summons Ho-Oh onto the scene, she attempts to use it to force Green to hand over the two feathers. Karen knocks her onto the ground, as Will reveals to Green that Silver was defeated easily by the two of them. As Green runs over to Silver and prevents him from falling, Karen asks whether their touching moment has ended. Enraged, Green stands her ground against the advancing Ho-Oh, much to Karen and Will's surprise.
Karen orders Ho-Oh to circle back and attack Green, but the attack is countered by Zapdos, Moltres and Articuno. She instantly takes to Ho-Oh's back, as Will does the same with Lugia. Karen then attacks the three birds with Ho-Oh, as the opposing factions engage in battle with one another. When Red and Green arrive on the scene, they assist Green in her battle against Karen and Will. Before long, Green notices that Karen is no longer riding atop Ho-Oh, to which Karen reveals that she was clinging onto Moltres' back the entire time. Karen then uses her Umbreon's Moon Light to send Green's Snubbull hurtling back to Green. She mocks Green's current situation, who lurches precariously off Moltres' back.
After the battle, Karen rescues an unconscious Will, flying away with aid from her two Murkrow. They are then approached by Koga and Bruno, who invite Karen and Will to join their organisation.
HeartGold and SoulSilver Chapter
Karen reappears alongside her teammates at Johto's Pokéathlon Dome, announcing the reformation of the Johto Elite Four.
Umbreon was initially seen as a Eevee during the flashback where Green and Silver attempted to escape from the Masked Man's base. It was ordered by Karen to attack the escaping pair, leaving Silver grievously injured. It later appeared as a fully evolved Umbreon at the Pokéathlon Dome alongside Karen and the other Elite Four during the HeartGold and SoulSilver Chapter.
Ho-Oh was granted to Karen by the Masked Man in order to prevent the Pokédex Holders from proceeding past the Ilex Forest Shrine. However, due to the combined efforts of the Pokémon and the Pokédex Holders, both Ho-Oh and Lugia are freed from Karen and Will's control.