Rat Magic

Amy Madison as a SHS student in Buffy's year. Willow and her used to be great friends but the drifted apart. In 'Witch' Amy takes part in the cheerleading tryouts with Buffy, and Willow finds her a little different, but thinks nothing of it. Buffy & Amy don't make it on the squad and when the girls that did start having suspect accidents, the gang figures out she's a witch. But when they meet her mother, they realises, she's the witch and has switched bodies with her daughter to relive her glory days as a teen.

Buffy: Amy? Are you Amy?
Giles: I don't understand?
Buffy: She switched! She switched your bodies didn't she!
Giles: Good Lord!
Buffy: She wanted to relive her glory days.
Amy: She said i was wasting my youth. So she took it.

Giles manages to re-switch them back into their rightful bodies, but Catherine goes berserk and attacks Buffy with a spell. Buffy manages to reflect the spell using a mirror, leading Catherine to disappear. Amy moved to her father's house, and magic was clearly the last thing on her mind...

This Saturday night he wants me to stay in and make brownies. Well, the brownies were my idea.
    - Amy, the Witch

But a year later, she's using magic on her teachers and Xander catches her, and sees it as the perfect way to get back at Cordelia for dumping him. But it turns out her magic isn't that efficient and the spell backfires.

Xander: Amy, good to see you. You're a witch.
Amy: No I'm not. That was my mother, remember?
    Bewitched, Bothered and Bewildered.

During the same episode, Buffy and Amy argue, and Amy turns Buffy into a rat. She uses the same spell a year later. Caught during a witch hunt by crazed parents, they prepare to burn her alive at the City Hall. She turns herself into a rat to escape.

Amy: You wanna fry a witch? I'll give you a witch! Goddess Hecate, work thy will!
Buffy: Uh-Oh!
Amy: Before thee let the unclean thing crawl turns into a rat.
    - Gingerbread.

Willow takes care of her and despite several attempts, fails to de-rat Amy until Something Blue, when she performs a spell to have her will done.

Willow: The only real witch here is fuzzy little Amy.
Buffy: I think you're being too hard on yourself.
Willow: She's got access to powers I can't even invoke. I mean first she's a perfectly normal girl - the rat turns into Amy - then poof! She a rat! - Amy turns back to the rat.
    - Something Blue

Willow finally manages to un-rat her in season 6 episode 'Smashed'. Amy is a little disturbed about coming back.

Amy: Everything feels so weird. I mean, it's like... I felt i was in that cage for weeks. But it can still be okay... right? I-I can still get into the swing of things, like... prom's coming up! I-I'm so hoping Larry would ask me. We would make such a splash at - Oh, Oh God. He hasn't asked someone else, has he?
Willow: Uh, Amy, three things we have to talk about. One, Larry's gay. Two, Larry's dead. And three, high school's... kinda over.
Amy: How long was I in that cage? How long?

But Amy soon gets herself together and hits the Bronze with Willow and the two use magic rather recklessly.
Amy also shows Willow some pretty shifty places and some pretty shifty people, pushing Willow even deeper in her magic craze spiral.
When Willow realises she can no longer control herself, she decides to no longer hang out with Amy and informs her she now refuses to use magic. Upset, Amy tries to tempt her by cursing her, but Willow resists, and firmly tells her to leave her alone.

A year later, Willow realises Amy is worse than she though. Jealous of Willow's powers, she casts a spell that lets the victim choose their own penance, and generally turns out to be something the fear greatly. And in Willow's case, it's turning into Warren.

Kennedy: Why would you do this to her? You really hate her that much?
Amy: This is not about hate. It's about power. Willow always had all the power, long before she even knew what to do with it. Just came so easy for her. The rest of us - we had to work twice as hard to be half as good. But no one cares about how hard you work. They just care about cute, sweet little Willow. They don't know how weak she is. She gave in to evil - stuff worse than I can even imagine - she almost destroyed the world! And yet everyone keeps on loving her. So what's wrong with having a little fun, huh? Taking her down a peg or two.
    - The Killer In Me