Melanie is a newspaper heiress, a rich socialite with enough time on her hands to get into plenty of trouble. As the film progresses, her glamorous façade is slowly stripped away until our golden girl is a disheveled, catatonic wreck.
The upside? She probably becomes a better person—if she ever recovers.
In our first scene with Melanie Daniels, she's strolling down the street in San Francisco, dressed to the nines. Some guy wolf-whistles at her; she likes it.
We're already not sure we like this gal. And then, the first thing she actually does in the movie is lie.
She's in a bird shop when Mitch Brenner comes in, and she pretends to be a saleswoman. "Just what is it you're looking for, sir?" she coos, and then proceeds to pretend she knows anything at all about birds, which she doesn't. Then, when Mitch turns the tables on her and tells her he knew she was faking all along, she gets cranky and tells him he's a "louse."
So, there's Melanie: she's irresponsible and spoiled and kind of a jerk. It's no surprise to find out later that she was in all the newspapers for jumping naked in a fountain while on vacation in Italy. She's got the money and the disposition to do whatever she feels like doing—even if that means motoring up to Bodega Bay on a whim in order to play a practical joke on Mitch.
Bottom line: Melanie is used to getting what she wants. She calls up a connection at daddy's newspaper to run down Mitch's license plate and get his home address. Then, she goes into his apartment building with a cage of lovebirds. When that doesn't work, it's off to Bodega Bay for a drive-by bird dropoff. That's chutzpah.
Melanie arrives in Bodega Bay in search of Mitch and still feeling pretty mischievous. Clearly way out of her element, she depends on the locals to find out where Mitch lives and even shows up at Mitch's ex's place to get some info about his sister. They all don't know quite what to make of this glamorous city girl, but Melanie manages to pilot a small boat well enough to make it to Mitch's house, walk inside, and deposit the lovebirds.
The turning point in Melanie's character arc? Fittingly, it's bird-related: when she gets knocked in the head by a gull on her way back from Mitch's. This brings Mitch into the picture, and as they start to build a relationship, we see a different side of her. She's still flirty and sarcastic, but Mitch presses her about her past, and she owns up to it. She defends herself but admits she hasn't been the nicest kid on the block.
MITCH: You were just a poor, innocent victim of circumstance, huh?
MELANIE: I'm neither poor nor innocent, but the truth of that particular—
MITCH: The truth is you were running around with a pretty wild crowd ...
MELANIE: Yes, but—
MITCH: ... who didn't much care for propriety or convention or ...
MITCH: ... the opinions of others, and you went right along with them, isn't that the truth?
MELANIE: Yes, that's the truth. But I was pushed into that fountain, and that's the truth, too.
At Cathy's birthday party, we get the backstory about Melanie; she opens up to Mitch about her family and tries to explain that she's really more serious and decent than she seems—she takes classes and does some volunteer work.
MELANIE: You see, Rome … That entire summer, I did nothing but … Well, it was very easy to get lost there. So, when I came back, I thought it was time … I began … I don't know … finding something again.
She even talks about her sad past:
MITCH: You need a mother's care, my child.
MELANIE: Not my mother's.
MITCH: Oh, I'm sorry.
MELANIE: What do you have to be sorry about? My mother? Don't waste your time. She ditched us when I was 11 and ran off with some hotel man in the east.
At this point, Melanie becomes a bit more sympathetic—the cliché of the poor little rich girl.
Rising to the Occasion
Once the bird attacks begin in earnest, Melanie seems to rise to the occasion. She's as terrified as anyone, but she's able to help the family deal with the chaos, and she comforts Cathy and Lydia.
But, don't take our word for it. Here's what Hitchcock himself had to say about her:
Generally speaking, I believe that people are too complacent. People like Melanie Daniels tend to behave without any kind of responsibility, and to ignore the more serious aspects of life. Such people are unaware that catastrophe surrounds us all. But I believe that when catastrophe does come, when people rise to the occasion, they are all right. Melanie shows that people can be strong when they face up to the situation, like the people in London during the wartime air raids. The birds basically [symbolize] the more serious aspects of life. (Source)
Melanie, like everyone else, struggles to understand why these bird attacks are happening, but she seems to think there's some kind of intention behind the attacks. When the folks in the diner are trying to challenge that idea, she insists the birds are out to get them:
SEBASTIAN: What do you think they were after? Miss, uh—
MELANIE: Daniels. I think they were after the children.
SEBASTIAN: For what purpose?
MELANIE: To … to kill them.
MELANIE: I don't know why.
Don't worry, Mel; nobody knows.
Melanie Melts Down
After the explosion at the gas station, one of the women blames Melanie for the attacks. They only occurred after the evil hussy arrived in Bodega Bay, after all. (That's not really true, but she's the easy target.) Despite the accusations, Melanie continues to try to help out, going with Mitch to retrieve Cathy and comforting Cathy after Annie's death. She stays in the house with the Brenners during the next assault, and after things have quieted down, she even goes upstairs to investigate some noises.
Um, didn't she see Psycho?
It's the beginning of the end for our heroine. She's madly attacked by crows and gulls; by the time Mitch and Lydia drag her out of the attic, her fancy clothes are torn, she's bloodied all over, and her perfect hair is a perfect disaster. She's totally come apart.
They carry her downstairs, and she's in a daze. She flaps her hands at imaginary birds; she can't speak. The family realizes they have to get her to a hospital, and they slowly walk past the hordes of sitting birds, get in their car, and drive away. She's still got her fur coat on—a little statement about how far she's fallen.
Remember how Annie told Melanie that Lydia is her good friend now that Annie was no longer dating Mitch? Our last scene is Lydia looking lovingly—and maternally—at Melanie, who's no longer a rival for Mitch's attention.