Girl Posse: They're the titular mean girls that rule the school.
Large and in Charge: The two dominant members (Cady and Regina) are noticably taller than the two followers (Gretchen and Karen.)
Politically Incorrect Villain: They have no qualms about branding Janis a "dyke," and Regina uses the word "retarded" casually in her first on-screen conversation.
Popular Is Dumb: Played straight with Gretchen and Karen (especially Karen). Zig-zagged with Regina who despite being able to manipulate most of the female half of the school into becoming her pawns, frequently shows herself to be rather Book Dumb.
Eating Lunch Alone: On her first day at school, she ends up eating lunch in a bathroom stall because she doesn't know anyone. After everything blows up in her face, she eats lunch in the bathroom again because everyone knows her, and they hate her.
Face-Heel Turn: Over the course of the film, the friendly and good-hearted Cady usurps Regina's position and becomes the mean-spirited Alpha Bitch of the school. She goes through a subsequent Heel-Face Turn when she's confronted with her actions.
Fish out of Water: To the point where Regina describes her as both "like a Martian" and "socially retarded and weird."
He Who Fights Monsters: After a while, Cady is no longer pretending to be like the plastics, she's genuinely acting like one.
Home Schooled Kids: Until the beginning of the movie, Cady had never gone to a regular school and had been homeschooled since she was around four years old.
The Mole: Janis convinces her to infiltrate the Plastics so she can spy on Regina and take petty revenges on her.
Na´ve Newcomer: She has no clue how to function or fit into the social climes of highscool. She's taken under Janis's wing, and then Regina's.
Nerds Are Sexy: Zigzagged. Kevin likes her because of her looks and her math skills. Aaron tells her joining the mathletes is social suicide and she dumbs herself down to impress him.
No Social Skills: Being homeschooled on a different continent makes for a serious lack of understanding how highschool politics work.
Rich in Dollars, Poor in Sense: Regina's a specific example. She could rattle off several mostly-illegal weight loss drugs (most of which either were expensive before being banned or would be expensive to ship from countries where they're still legal). Yet, she didn't know enough about health and nutrition to know whether or not butter is a carb. This is most likely a jab at the stereotype of American teenage girls wanting to get thin or in shape the fast and unhealthy way than through actual healthy living.
Shrouded in Myth: An interview montage with other students and even some faculty highlights the silliest myths and rumors about how cool/mean Regina is.
Smug Snake: Regina is damn proud of every little manipulation that she pulls off.
Spoiled Brat: She regularly tells her mother to shut up or leave her alone. She also has the master suite in her house because she forced her parents to switch rooms with her.
Janis: Gretchen Wieners knows everybody's business, she knows everything about everyone.
Damian: That's why her hair is so big, it's full of secrets.
Beta Bitch: She appears to be this to Regina initially.
Catch Phrase: She desperately wants to start one: "That is so fetch!"
Everyone Is Christian At Christmas: Despite the fact that Gretchen's Jewish, she performs Jingle Bell Rock, which is a Christmas song, with the other Plastics every year for the Winter Talent Show.
Extreme Doormat: She lives to do Regina's bidding. Even after the breakup of the Plastics, she doesn't seem to be able to function without someone to tell her what to do, and finds another queen bee to serve.
Mixed Ancestry: She's Jewish, but joins the Cool Asians at the end of the movie.
Neologizer: Primarily in the form of "fetch". Which will never catch on.
Screw the Rules, I Have Money!: Gretchen's ticket into the Plastics is that she's rich. When they get called to the principal's office for the burn book, she specifically tries to use her money and her father's notoriety to get out of trouble.
Janis: That one there, that's Karen Smith. She is one of the dumbest girls you will ever meet. Damien sat next to her in English last year.
Damian: She asked me how to spell orange.
Advertised Extra: Karen doesn't have that large of a part (especially compared to the other three plastics) and has little impact on the plot, but is featured in all of the advertisements because she is a Plastic and thus one of the Mean Girls.
Blondes are Evil: Subverted; she's pretty nice mostly and is easily the least malicious of the Plastics (Cady included.)
Informed Ability: Apparently, her breasts can tell when it's going to rain. Or at least, when it's already raining. And even then, she's not completely sure, even if she's actually OUTSIDE in the rain.
The Pollyanna: As well as being the least malicious Plastic she's easily the happiest too, lacking Regina's rage, Gretchen's crippling neurose, or Cady's need to fit in. She's even aware of (and comfortable with) the knowledge she's not very bright.
"There are two kinds of evil people in this world. Those who do evil stuff and those who see evil stuff being done and don't try to stop it."
Cady: You know what? You're the one who made me like this so you could use me for your 8th grade revenge!
Janis: God! See, at least me and Regina George know we're mean! You try to act so innocent like, "Oh, I use to live in Africa with all the little birdies, and the little monkeys! [...] you are a mean girl! You're a bitch!
Ambiguously Gay: Is Janis Ian a lesbian? Damn hard to say. It's proven she's not a lesbian when she starts dating Kevin at the end of the movie.
Important Haircut: Offscreen. After Regina spread the rumor in middle school that Janis was a lesbian, Janis dropped out of school for the rest of the year; when she returned in the fall, "all her hair was cut off, and she was totally weird."
Karma Houdini: Janis, who, despite influencing and encouraging Cady to join the Plastics specifically to damage Regina, gets zero comeuppance when she reveals it to the entire crowd of girls following the revelation of the Burn Book.
Manipulative Bitch: Janis convinces Cady to join the Plastics just so she can information from them.
Broken Bird: She's fresh off a divorce and you can see she's still fragile from it.
Cool Teacher: She's young and genuinely cares about her students. Once Cady stops being a plastic, she starts realizing how cool Ms. Norbury actually is.
Cady: (stating why she can't go to the dance) I'm grounded.
Norbury: You're already out.
Clear My Name: During the intervention, she confronts Cady and asks if there's anything specific she would like to confess. Cady refuses. Ms. Norbury ends up the subject of a police investigation as a consequence.
Disappointed In You: Says this word-for-word when Cady initially refuses to admit she wrote the "drug pusher" page in the Burn Book.