Katy Jensen is the hidden villainess of "Secret Admirer", book #36 of R.L. Stine's original Fear Street series (published on March 1, 1996). She was the best friend of protagonist Selena Goodrich, who was previously a shy and overweight student. But by the events of the book, Selena had lost weight and become the most popular actor in Shadyside High--while Katy remained fairly unpopular as a stagehand for the drama club. Resentful over Selena abandoning her for fame (in her mind), Katy began sending Selena frightening notes and "presents". She did so under the alias "The Sun", a play off Selena's nickname, Moon, and decorated all the things she sent Selena with a sun sticker.
Katy's motivation for tormenting Selena was to scare her out of performing in the school's final play of the year, which was a production of Romeo & Juliet with Selena as the female lead, believing it would make her and Selena close again. Her schemes became more violent, including pushing a cabinet over onto Selena's understudy Alison Pearson (having thought Selena was sitting there) and trying to run Selena over with her car while she was out on a date with Eddy Martin, a college intern who was supervising their latest production. To deflect suspicions from herself, it's implied that Katy even set herself up to be almost crushed by a set of stage lights she rigged to fall.
When Selena confided in her friend Jake Jacoby about her stalker, he began investigating and learned that Katy was behind everything after finding a sheet of sun stickers in her auditorium locker. After learning from Selena that Jake was waiting at the auditorium to tell Selena the truth, the evil Katy went ahead of her and lured Jake up on the catwalk before shoving him to his death to keep him quiet, with his death later ruled an accident. To Katy's benefit, Jake's act of stashing her stickers in his own locker led Selena to believe that Jake was her stalker and that she was now safe. But late one night, Katy planted another note in Selena's house revealing that her stalker was still on the loose. Katy nearly accomplished her goal when Selena decided to drop out of the play, but was convinced by her ex-boyfriend Danny Morris when he pointedly told her how many people she'd been hurting if she dropped out. Angered, Katy placed another note in Selena's locker, saying that she was going to have killed someone close to her by the following morning and planned to kill her during the play's opening night. That night, Katy disguised her voice as Selena's to lure Danny to the auditorium costume room that night. Once there, Katy hit him in the head with a flashlight before tying him up and placing him behind a cabinet.
Selena arrived at the auditorium soon afterwards, having found the note earlier than Katy planned and rushed over under the belief that Katy was her stalker's next victim. But once Selena discovered Danny in the costume room, Katy soon after came into the room. Katy initially feigned confusion before revealing herself by knocking Danny out with the flashlight. The psychotic Katy then went into a rant against Selena, accusing her of being self-centered and boasting about how easy it was to convince her that someone else was after her. Despite Selena's pleas, Katy attacked Selena and the two girls got into a fight on the catwalk, where Katy attempted to push Selena off. But before she could, Eddy arrived (having called Selena right before she left for the school and noticed how upset she sounded) and tried to dissuade Katy from killing them. But Katy ignored him and attacked, knocking him off the catwalk and leaving him hanging on to the edge. When Katy began hitting Eddy's fingers with the flashlight to get him to fall, Selena managed to tackle Katy and held her off long enough for Eddy to get back up and help her subdue the insane villainess. After that, Eddy took Katy into the costume room, untying Danny and using the ropes to restrain Katy. Katy was presumably arrested later on for murdering Jake and attacking Selena and Eddy.
- "You're so conceited, Selena. It was so easy to convince you that a stalker was following you around. Even when there was an accident, I could make you believe some crazy guy did it! I mean, a ladder falls down and you think you have a stalker! All I had to do was put a sticker on it! It's obvious all you care about is having guys fall at your feet." (Katy's evil boasting to Selena)