\\I drank the potion she offered me
I found myself on the floor
Then I looked in those big green eyes
And I wondered what I came
there for.\\
They meet her the first time when they're digging for information in the library. She's a librarian, very friendly, very helpful. The first thing Dean thinks when she strolls up to them with a cheerful "May I help you?" is You can certainly help me.
Both she and Sam give him a look when he immediately voices the thought. What? he mouths, shrugging at them. Dean Winchester is not known for keeping things to himself. She smiles winningly at Sam when he apologizes for his brother's behaviour, and helps them find articles in the town's old newspapers.
The second time they meet her she's crouched over a lifeless body in an abandoned warehouse near the sea. She smiles at them, and suddenly they're faced with the fact that the warehouse is indeed inhabited by something-- multiple somethings, in fact.
She makes her escape while the brothers are fighting to fend off a multitude of crows, toads, and cats-- "Her familiars," says Sam, who finally realizes why there have been both mysterious human disappearances and an influx of animal life in the area. "I can see that, Sam," Dean shoots back, annoyed at being bested by one girl and a bunch of dirty, noisy animals-- and Dean can only shoot uselessly into the looming animal life, cursing the bitch.
The third time she appears she's got Sam under some sort of spell. Dean recognizes the marks she's written in chalk around his brother's prone body from the last time they met, and he remembers the lifeless eyes of the dead guy, whose soul, according to Sam, had been transferred to an animal's body so that she could siphon off power for her magik as she needed it. A goddamn magical battery, Dean thinks to himself, and damned if he's gonna let his brother become her equivalent of the Energizer Bunny.
"She's a human; she's just somehow tapped into the latent supernatural energy here," is what Sammy told him earlier, but all Dean can see is his little brother laying there, too out of it to defend himself. That's all right though. That's what older brothers are for.
She's dead before she even hits the ground, with a silver bullet through her heart.