Jenny is a computer science teacher at Sunnydale High whose modern approach does not sit well with Rupert Giles. She calls herself a techno-pagan and is a member of a cyber-coven. Jenny assists the Scooby Gang in the first two seasons of Buffy. As they work together against evil, Jenny and Giles gradually fall in love.
Jenny first appears in the episode "I, Robot... You, Jane" as a computer science teacher, whose teaching methods and interests are at odds with, and seem to infuriate, Giles. She later appears in the season finale "Prophecy Girl," approaching Giles after she becomes anxious over several signs of an impending apocalypse she's become aware of. Giles soon reveals the truth about the Master's plan and Buffy's true identity. Jenny is present when the Hellmouth is briefly opened during Buffy's battle with the Master.
Throughout the earlier stretch of the second season, Jenny's flirtation with Giles gradually becomes a slow-burning romance. However, the romance is hindered in the episode "The Dark Age" when Giles' past comes back to haunt him, and Jenny is possessed by a demon Giles had summoned in his youth. In the aftermath, Jenny avoids Giles for some time, but by the episode "Ted," she is finally ready to forgive Giles, and by the end of the episode, the two of them have rekindled their romance.
Buffy loses her virginity to Angel in the episode "Surprise," in which Jenny is revealed to be Janna of the Kalderash gypsy clan, which had cursed Angel with his soul and years later sent Jenny to watch over him. Jenny was not told that Angel would lose his soul, but the Scooby Gang is still angry that she had concealed her identity. She decides to make it up to Buffy by attempting another restoration and translates the spell using a computer. However, Drusilla senses Jenny's plan and warns Angelus, who kills Jenny before she can cast the curse. Angelus leaves Jenny's dead body in Giles' bed with a trail of romantic symbols leading to the scene (cf. "Passion"). Giles is devastated and seeks revenge. Xander views Jenny's murder as evidence that Angel is beyond redemption. After Buffy rescues Giles from his ill-conceived act of revenge and visits the grave of the ill-fated Jenny (buried under her assumed name of "Jennifer Calendar"), Buffy reveals that due to the loss, she is finally ready to slay Angel. However, Willow eventually finds Jenny's diskette containing the spell to restore Angel's soul in "Becoming, Part One".
In the episode "Becoming, Part Two," Drusilla manipulates a badly tortured Giles into seeing her as Jenny and revealing what is needed to awaken the demon Acathla.
In the third season episode "Amends," The First Evil assumes Jenny's form to torment Angel into killing Buffy (or himself, it does not care which). After this, Jenny is not seen or even mentioned again for the remainder of the series' run.