Service Sector Stereotypes: Poor, poor "Diner Girl." Somewhat justified in that her stepmother is taking advantage of the situation.
Tomboy with a Girly Streak: Being raised by a single father, Sam grows up knowing more about sports than fashion, though this is more of an Informed Ability at best. She has no aversion to wearing a gown for the ball, is initially upset to learn her mystery boy is a jock, and is more interested in academics than sports. She also loves fairy tales.
Good Adultery, Bad Adultery: He is essentially cheating on Shelby with Sam from the beginning. This is portrayed as a good thing as Shelby is "evil."
Jerk Jock: Played with. He tries to be the Nice Guy and the movie expects us to sympathize with him. He also wants to recreate himself as a writer. On the other hand, he cheats on his girlfriend, practically stalks Sam, rejects her for being a waitress, and makes no effort to stick up for her to Shelby. He does not even appear to help Sam reclaim her inheritance back at the end. It's kind of a Broken Aesop that his only redeeming characteristics are that he is rich and pretty.
To his credit, it is implied that he never actually rejected Sam, he was just unsure what to do in the situation. Sam certainly believes this, as why else would she tell him that she was not going to wait for him anymore. For what it's worth, he never goes back to Shelby.
Nice Guy: He actively tries to be this, but has issues.
Bitch in Sheep's Clothing: Not to the viewer, but it is implied she gets away with exploiting her stepdaughter because she is merely helping her to raise money for Princeton. Little does Sam know that Fiona has no intention of getting rid of her unpaid employee.
Conspicuous Consumption: How the hell she is able to flaunt so much is never explained when she would logically have as much as Sam's father, but with even more dependents. Of course, she is at least saving money by extorting Sam, and is probably mooching off her husband's insurance (though this is never mentioned.)
Fostering for Profit: Unusually, her victim, Sam, is her stepdaughter rather than a foster-child, but the trope still applies, as she extorts the money Sam makes at the diner for herself. To make matters worse, she also stole Sam's inheritance. Sam could have saved herself years of abuse and humiliation if she just looked in her old storybook.
Fortunately, she and her daughters are forced to work off every last cent they took from Sam, including the money Sam earned for them as a waitress.
Gasshole: Brianna. As if farting (while in the pool) in her sister's general direction wasn't enough, during the "race" back to the diner, she says (out loud, and in a cat costume, no less) that she needs the "litter box".
Spoiled Brat: Both; their spending bites them on the ass when they are forced to pay off their debts for the money they stole from Sam.
Those Two Guys: They practically don't do anything until three-quarters into the movie.
Too Dumb to Live: Brianna, who even forces Sam to do her homework for her. Gabriella is a bit craftier.
Wounded Gazelle Gambit: They lie to Shelby that Sam set up the whole "Cinderella" game in an attempt to hurt her, and add on that they only waited to tell her because she threatened them.
Absurdly Powerful Student Council: How the hell does she go unpunished for the pep-rally scene? Seriously, the faculty are visually mortified at what she did to Sam, but they seem to have forgotten they have the power to punish her.
Alpha Bitch: She not only mocks and belittles Sam for presumably years, she seems to have turned the whole school against "Diner Girl."
Evil Is Petty: Let's make fun of the girl who has to work for a living! Borders on Fridge Horror when you realize she does not have a problem with Austin being a gas-station attendant (who like Sam, works for his family's business, they are just take advantage of the situation like Sam); when you think about it, she essentially tortures Sam for being an abuse victim.
Expy: Being a bitchy blonde who tortures Sam, she is practically a teenage version of Fiona. The movie highlights this by having her be followed around with a red-haired and a brunette lackey.
The idea of giving the prince an evil fiancee seems to have become more popular in recent literary Cinderella retellings. Queen Levana in The Lunar Chronicles or Lady Rainhilda in The Captive Maiden seem to be based on her, especially the latter.
Hate Sink: Shes needlessly cruel to Sam and the pep-rally scene will surely get her underneath your skin.
Karma Houdini: She goes completely unpunished for torturing and humiliating Sam.
Technically, she does lose her rich boyfriend to Sam, and he ends up rejecting her in public, but he intended to do this since the beginning. Carter does the same in the epilogue, but once again it is completely unrelated to her abuse of Sam and only because she rejected him earlier.
Kick the Dog: If her rejecting Carter was not enough for you, the pep-rally scene kills what little sympathy you might have for her.
Popular Is Dumb: Has to cheat off a "geek" to pass tests, only thinks that The Three Musketeers are chocolate, and doesn't even know what a foreign word for "goodbye" means.
To top it off, she and her posse were stupid enough to even believe the lies that the stepsisters told her about Sam wanting to steal away Austin.
Rich Bitch: Probably, though the movie never actually mentions that she has money except in a deleted scene, but this is presumably why she looks down on Sam, who was to work for her keep. It is also possible, however, that she is merely a Gold Digger, as she is clearly aware of Austin's money and status.
Took a Level in Jerkass: While she is bully from her first appearance, the movie sets her up for a HeelFace Turn when she shows interest in Carter. She later snobbily rejects him in public when she realizes he is an outcast boy, and when she realizes Sam is "Cinderella," she publicly humiliates her at the pep-rally scene.
Ungrateful Bitch: To Carter. He is probably the first guy to ever treat her like a lady, but later wants nothing to do with an outcast like him. After Austin finally leaves her for Sam, Shelby makes another pass at Carter (just because he's moderately famous now), who in turn leaves her for Astrid.
Even Evil Has Standards: They never try to trick Austin into thinking they are "Cinderella," though this is probably more out of a fear of Shelby than anything.
Of course, he probably already knows it is not one of them.
Expy: They seem to be extensions of the "evil sisters." They have a similar color scheme to them.
Green-Eyed Monster: Lampshaded by Shelby when Madison tells her to quit ranting about the guy (Carter) who saved her during the costume party.
Shelby: "Madison, I think there's a green monster on your back called 'Mr. Jealousy'!"
Karma Houdini: They gleefully help Shelby torture and humiliate poor Sam, as if she did not have enough problems, and Carter throughout the movie. The closest they get to a comeuppance is when Carter rejects the three of them when Shelby makes an unsuccessful pass at them. It is possible that they were also trying to flirt with him, or that the whole thing was a last-ditch attempt to save face.
Rich Bitch: Similar to Shelby, this is probably the case, but this is unconfirmed in the final movie.
Sassy Black Woman: She is usually a friendly, caring person, but she will get sassy if you deserve it. Partially subverted in that she puts up with Fiona for Sam's abuse.
Screw the Rules, I Have Connections!: She is able to get away with going against Fiona's abuse because she practically runs the place, and especially brings in the clientele. She manages to get the entire diner to walk out on Fiona in Sam's defense towards the end.
Service Sector Stereotypes: Played with. Being a waitress is actually portrayed as a decent job for Rhonda when she worked for Sam's father, and only became demeaning when Fiona took over. Played straight in that her happy ending is being promoted to "co-owner."