Official Couple: With Joe, until he breaks up with her because he believes she's cheating on him with Martha (Or, in These Three, because she believes he's cheating on her with Martha). In These Three they end up together in the end but other versions end on No Romantic Resolution.
Tomboy and Girly Girl: She's the girly girl to Martha's tomboy. Lampshaded in the 1961 adaptation when Martha says that she's fine with plain clothes but Karen likes to be dressed up.
Two-Teacher School: Karen and Martha are the only two teachers at the all-girls boarding school they own.
Adaptational Angst Upgrade: Inverted in These Three. She doesn't suffer from nearly as much stress and trauma in the 1939 film as she does in the play. This largely due to, despite being a huge scandal, being Mistaken for Cheating with a man wasn't as scorned as being mistaken for cheating with a woman.
Adaptation Dye-Job: Is normally a brunette but is played the blonde Miriam Hopkins in These Three.
Driven to Suicide: She hangs herself (shoots herself in the play) after confessing to Karen near the end.
Forced Out of the Closet: Accidentally done by Mary. She was lying, but her accusations of Martha being in love with Karen were actually true. This turns out terribly because Martha wasn't remotely ready to come out.
Gayngst: Martha suffers from a lot of repression and angst, not helped by the fact she felt that her feelings ruined her and Karen's lives.
Gayngst-Induced Suicide: Martha kills herself near the end soon after confessing to Karen. In her case, it was due to the troubles over her sexuality combined with everything that had occurred to her.
Stepford Smiler: Even early on she kept a facade of happiness despite being stressed over Karen loving Joe, not her. It gets worse as the story goes on. In the end she's dropped all of her facade, though she tries to bring it back up a little right before she committed suicide.
Tomboy and Girly Girl: She's the tomboy to Karen's girly girl. Lampshaded in the 1961 film when she says that she's a "skirt and blouse" person while Karen likes dressing up.
Tomboyish Voice: Some actresses, such as in These Three and the BBC radio version, portray Martha with a deeper voice than Karen.
Tomboy with a Girly Streak: She does most of the housework around the school despite being the more tomboyish of the two teachers.
Two-Teacher School: Karen and Martha own an all-girls boarding school. They're literally the only two teachers around.
Drama Queen: She makes a lot of commotion out of nothing. At one point she pretends to be sick and faints just to get out of a scolding. She's also prone to crying when things don't go her way.
Enfant Terrible: She lies constantly, slaps her classmate Rosalie, and blackmails her.
From the Mouths of Babes: It's implied she doesn't quite understand the implications much, but it's made clear enough that Mary said she heard Martha and Karen being intimate.
Ironic Name: She's a spoiled, drama queen named "Mary".
Karma Houdini: While her lies are uncovered, she never gets any real punishments for what she did. The most that occured was her being spanked by the maid in These Three and getting glared at in the other versions.
Malicious Slander: She spreads rumors that Martha and Karen are having an affair in order to get out of going to school.
Missing Mom: Her mother's whereabouts are never mentioned.
Parental Abandonment: Her parents are never referenced in the films. In the play her father is dead but her mother is never mentioned.
Raised by Grandparents: Her parents whereabouts are unknown in the films, though it is mentioned her father is deceased in the play. Her grandmother seems to be her primary caretaker.