"Strong Pokémon. Weak Pokémon. That is only the selfish perception of people. Truly skilled trainers should try to win with their favorites. I like your style. You understand what's important. Go on—the Champion is waiting."
— Karen, Pokémon Gold, Silver, Crystal, HeartGold and SoulSilver.
There are two different sides to Karen in the games she is seen. She is a kind-hearted and skilled Pokémon trainer who specializes in dark types. Her policy was there's no such thing as weak or strong Pokémon, people should try to win with their favorite, in her case, her Umbreon. She was the second strongest trainer in Johto. Her party was Umbreon, Vileplume, Gengar, Murkrow, and Houndoom. In HeartGold and SoulSilver she returned with the same attitude and same party but had new moves and a new outfit. When defeated, she will use a different team consisting of Umbreon, Houndoom, Spiritomb, Absol, Weavile, and Honchkrow.
In the manga she was one of the masked children kidnapped at birth by the mask of ice and trained to do his bidding and she was cruel, mean and psychotic. Her only Pokémon were her Umbreon and two Murkrows.