Inca Mummy Girl

Sunnydale's exchange program brings students from all over the world to the school, and Joyce Summers is among the parents who volunteered to house the foreign kids. Buffy is not relishing the thought of sharing her house - nor is Xander when he learns that Ampata Gutierrez from South America is a boy.
After a field trip to a local museum, a young student tries to steal the seal on an Incan princess's mummified remains, and breaks it. This awakens the mummy, allowing her to drain the life, first from her would-be grave robber, then from Ampata, who is waiting for the late Buffy to pick him up at the bus station. Assuming the identity of Ampata, the princess learns about the modern world - and she and Xander fall for each other, to Willow's chagrin. As Xander and Ampata slow dance at the Bronze, Buffy and Giles realise that the exchange student is an impostor.
        - Summary taken from 'The Watchers Guide, Vol 1'.

She was 16, like us. She was offered as a sacrifice and went to her death. Who knows what she gave up to fulfill her duty to others. What chance at love?
     - Ampata to Buffy.

Five hundred years ago, the Incan people chose a young sixteen year old girl and sacrificed her to the Gods to obtain their protection.
She was mummified and bound in a state of living dead by a ceramic seal clasped between her hands.
When the seal is broken, the mummy rises. By killing humans and sucking their lives, she renews her own and turns into a beautiful young girl. She poses as Buffy's South American exchange student and delights in living the life of a teenager with the gang.
Ampata's fate is associated with Buffy's. her guardian even refers to her as the Chosen One. her destiny was to give her life away to service others. Buffy is sympathetic of her fate, understanding what she went through.

Buffy: She was gypped. She was just a girl, and she had her life taken away from her. I remember when i heard the prophecy that i was going to die. I wasn't exactly obsessed with doing the right thing.
Xander: But you did. You gave up your life.

When the gang discovers who she is, they prevent her from killing and she rapidly decays, returning to her mummified state.

Xander: I have the worst taste in women of anyone in the world ever.
Buffy: Ampata wasn't evil. At least not to begin with, and... I do think she cared about you.
Xander: Yeah, but I think that whole sucking the life out of people thing would have been a strain on our relationship.