Karen Richards is Margo's best friend. She's a playwright's wife, which, in a world of actors, directors, and writers, makes Karen the closest thing to us: a normal person. She even calls herself "the lowest form of celebrity." That makes her the perfect POV character for us average Joes and Janes to understand this glamorous world.
Note to self: Where'd she get those fabulous pearls?
Even though she's in the theater only by marriage, as Addison says, she gets the action rolling when she invites Eve into the theater to meet her idol. Margo's not interested but Karen insists:
KAREN: You can't put her out, I promised... Margo, you've got to see her, she worships you, it's like something out of a book.
Karen lives to rue the day she stood up for Eve. Karen had no way of knowing what a schemer Eve was. No one did, and no one blames Karen for it. Her heart was in the right place, and it almost always is. Karen is intelligent and fabulous at all times. She defends her husband, saying his play is "a fine and distinguished play," and she's a supportive friend, always by Margo's side.
Margo trusts her enough that she can speak her mind. When Margo starts downing martinis at the disastrous party, Karen knows something is about to go down:
KAREN: We know you, we've seen you before like this. Is it over - or just beginning?
Karen has a moment of weakness, though. She gets a little tired of Margo treating her friends like "members of her supporting cast." She hatches an idea to force Margo to miss a performance, giving Eve a chance to go on in her place. A harmless prank—she sees Margo getting increasingly paranoid about Eve, so she wants Margo to see that everything's okay even if Eve is Margo's understudy. Karen reasons that, "Every now and then there is nothing I want to do so much as to kick [Margo] right square in the pants."
This plan backfires fast.
She drains the gas in the car on a weekend trip and sure enough, Margo misses her call. By the time they return to New York, they see that Eve had planned this all along, and as a bonus called every critic in town to dazzle them.
Things really hit the fan for Karen when Eve threatens to expose her, and to tell Margo that Karen drained the car on purpose. This would destroy not only her friendship with Margo, but her husband's career, too. Lucky for Karen, Margo quits acting to become a wife. Eve gets her role and Karen gets to keep her secret, but she never lets Eve forget it.
We last see Karen basically ignoring Eve at the award luncheon. She may have started as Eve's friend, but she's a sadder but wiser gal now.