Amy Madison was exposed to magic at an early age by her mother, who was a very powerful witch. When her parents' marriage broke up, her mother became obsessed with regaining her lost youth. When Amy went to High School, her mother decided that she was wasting her youth so she swapped bodies with her, forcing Amy into her mother's body and stealing the youthful, teenage body for herself.
Posing as a teenager, Amy's mother wanted to regain the title she had when she was in high school: head cheerleader, so she tried out of the squad at the same time as Buffy Summers. The two of them made the reserve team together, with Buffy ranked higher than Amy. But her sights were set on being on the squad so she started taking out team-members using spells until only Buffy was between her and the place she craved.
In her bid to be on the cheerleading team, Amy's mother cursed Buffy with a terminal disease that attacked her nervous system. Buffy quickly deteriorated, but Giles was fast to spot that it was a spell rather than a naturally occurring illness.
Giles managed to switch their bodies back, and cure Buffy's illness. Buffy was then well enough to deflect a spell Amy's mother was casting that trapped her in a cheerleading she had won in her cheerleading days. Amy was sent to live with her father which she enjoyed.
Amy was then later on, when Xander saw her hypnotizing a teacher into thinking she was handing in her homework. Xander who had just broken up with Cordelia, realized she was a witch, she had inherited her powers from her mother. He wanted her to cast a spell, making Cordelia fall in love with him again. The spell went wrong and every woman in Sunnydale fell in love with Xander. Except Cordelia. Amy was also affected by the spell. She turned Buffy into a rat when the two got into a fight over Xander. Giles managed to get her to reverse the spell and everything went back to normal.
She was then seen a year later, when two children were murdered by a demon. At the time she, Willow and a warlock named Micheal, were preparing a spell for Buffy's birthday that would make her birthday demon-free. However, a malevolent force was convincing the town that it was witches who were to blame for the deaths of the children. Amy, Willow and Buffy (who was also getting blamed), were going to be burned at the stake. Amy turned herself into a rat, it is still unknown if this was intentional or if she was trying to turn the crowd into rats. Willow kept her as a pet while she and Micheal tried unsuccessfully to turn Amy back into a human.
In season four, Amy was briefly turned into a human by Willow, who didn't realize she had temporarily gained the power to manipulate reality by what she said. But she turned her back into rat before anyone could notice.
In season six, Willow, who had become extremely powerful, conjures an incantation that turns Amy back into a rat. Amy thinks that she is turned back just in time for Prom (which was three years ago). Willow explains everything she missed, which shocks Amy. Amy and Willow become friends again.
However, Amy's personality has changed. She takes Willow to the Bronze and the two use their magic to cast spells for fun. When they return Willow has drained all her magic, but Amy takes her to see a warlock, who acts as a mystic drug dealer named Rack. He gives them both a power rush. This is when Willow's addiction to magic begins to get noticed by the group.
Willow puts Dawn's life in danger when she starts driving while on a magical high and the two are attacked by a demon. This makes Willow want to give up magic. But Amy casts a spell on her that makes her manipulate everything she touches. Willow says thios only makes giving up harder harder, Amy mocks her and claims it's revenge for not managing to turn her back quick enough. Amy later goes to see Willow, and Willow tells her that she is no longer welcome in the Summers home and cuts her out of her life.
It is then said that Amy saved Warren Mears after Willow flayed him and kept him alive using magic.
She is then seen in season seven with Willow's old coven when she attended UC Sunnydale. She appeared to have reformed after hitting "rock bottom", and apologised to Willow when Willow needed the coven's help. Willow had turned into Warren Mears, the man who had killed her girlfriend Tara, after kissing potential Slayer Kennedy. Amy later told Kennedy she cast the spell that turned Willow into Warren out of hatred. The spell is reversed when Kennedy kisses her.
It was revealed in season eight that Amy and Warren Mears (now her boyfriend), had been living in Sunnydale after it collapsed. They were discovered by the Army, lead by The General, and offered to take out Buffy. As long as Warren got acess to the weapons lab, and she got acess to all the Army's magical hardware and lots of cheese. Amy casts a spell on Buffy that can only be woken by love's true kiss, and sets and sets an army of the undead on the Slayers' headquarters. Willow confronts and after a magical battle, Amy is stomped on by a giant Dawn.
Amy then pulls Willow into a portal, at their base, where Willow is kidnapped. Buffy and Satsu go to recue her. Willow channels her power into Buffy, who makes Amy see her first fear, her mother. Satsu then throws a grenade at her but Amy gets out of the way. She then teleports herself and Warren away.
It seems that being turned into a rat, rejected by Willow and what Willow did to her boyfriend have all been factors in turning Amy from an ally into a bitter enemy.
Amy is a powerful witch with abilities inherited from her mother. These powers grow throughout the series. While initially more powerful than Willow, casting a spell to become a rat that Willow cannot initially reverse, Amy acknowledges in Season Seven that Willow has surpassed her. Willow comments that Amy's new found power in Season Eight (which she claims has grown since the events of Chosen. has been technologically augmented to some degree, with Amy commenting that she has grown consistently stronger stranded in the ruins of Sunnydale. In battle, Amy is able to stand up against Willow and fly by force of will.