Jenny Calendar(Robia LaMorte) or Janna was computer science teacher at Sunnydale High. At first we see her once in a while only as the cool computer teacher. By "Prophesy Girl," she finds out about vampires and she starts helping out the gang after that.
It's interesting and funny the way we see Giles take interest in her and Buffy try and help them out. They finally go out on a date and start being friendlier. It doesn't last long, however, when in "Dark Age" we learn about the way Giles(Ethan calls him Ripper) was when he was younger. He had brought forth a demon, Eygon, along with some of his friends. The demon had returned to kill all the ones who had participated in the ritual.
Eygon went from dead/unconscious body to dead/unconscious body. In an attack by the demon, Jenny had been unconscious and the demon had jumped into her body. No one knew until later on. That's when they called Angel.
When the demon was put in danger, it jumped out to the nearest dead or unconscious body. When Angel strangled Jenny, the demon jumped into Angel, since he's technically dead. Learning about Giles and the experience put a strain on her and Giles' relationship.
After that, we learn that Jenny is actually a gypsy from the Romani tribe that had originally cursed Angel. He job in Sunnydale was to keep an eye on him and to make sure that he never get content with his existance. She was never told the consequences of what would happen if Angel ever did get happy.
They all learned the hard way when she was unsuccesful in keeping him and Buffy apart and Angel lost his soul. Again, Jenny and Giles' relatioship is strained because everyone is angry at her for not warning them.
In "Passion," Buffy reluctantly forgives Jenny, and so does Giles. But Angel's alter ego decides to start playing with Buffy the hard way. Just as Jenny has discovered the original curse and is printing it out, Angel pays her a visit at the school. He runs after her and snaps her neck, then leaves her in Giles' bed. Right before he had killed her, though, he had distroyed the computer and burned the print-outs of the curse. What he had forgotten was the diskette she had saved it to. The disk was later discovered by accident by Buffy and Willow.
Jenny was never seen again until "Amends," when the First Evil takes on her form to torment Angel.