Dr. Leslie Winkle, Ph.D., is an experimental physicist at the California Institute of Technology who works in the same laboratory as Leonard Hofstadter. She is essentially his female counterpart, equipped with the same black framed glasses and zippered hoodies. Whenever depicted in the lab, she is always using the equipment inappropriately to prepare food, such as flash-freezing a banana with liquid nitrogen and then shattering it with a hammer to put in her cereal, or using a helium-neon laser to heat a cup of instant soup. She dates Leonard on-and-off, and later Howard Wolowitz. Because of her shallow and manipulative nature, she is arguably the central antagonist of the second season- although the only true hostility she ever displays is towards Sheldon Cooper. Leslie made her final appearance in "The Lunar Excitation" when she rejected Leonard's offer for casual sex and left the series after that
Leslie severely dislikes Sheldon as evidenced in most episodes. Leslie takes her job seriously (when she isn't using the equipment for food preparation) and is very intelligent; she figured out a problem Sheldon had been working on without difficulty, much to his irritation. She cares so much about science that it affects her love life, Her partner has to have the same scientific beliefs as her in order for them to stay together.
She is displayed as arrogant, condescending, and manipulative. When she and Howard started a "friends with benefits" relationship, she would only give him favors so long as he did as she wanted, admitting she could only be happy if she were the one in control. It is implied by dialogue (and said outright by Sheldon) that Leslie is promiscuous, as she suggested she tended to have orgies and mainly craved sex instead of a relationship. She wears the same type of glasses and clothing as Leonard does.
Leslie Winkle is an slender woman with fair skin, brown curly hair and eyes. She wore black framed glasses and a zipper hoodie jacket, pants and shoes in various colors (depending on which episode).
Leslie makes her debut in "The Fuzzy Boots Corollary." Leonard, feeling his pursuit of his neighbor Penny is pointless, asked her out on a date because he thought she was in his league. She kisses him to see whether they have any chemistry, and although complimenting his technique, turns the offer down when she feels no arousal. This incident, and his hopeless infatuation with Penny, causes Leonard to go into a state of depression. In her next appearance in "The Hamburger Postulate", she goes to Leonard's apartment to practice with her string quartet. She offers herself to Leonard to have intercourse, despite her previous rejection. She also inadvertently starts her rivalry with Sheldon when she solves a problem he was having and insults his intelligence. Assuming he and Leslie are now dating, Leonard goes to her only to find she just used him to satisfy her libido, which is apparently now covered until New Year's Eve.
When the guys compete in a physics bowl and kick Sheldon off the team for his showboating behavior, Howard and Raj want Leslie to join. Leonard is understandably uneasy since she had used him, but agrees. Leslie at first rejects, but reconsiders when she discovers Sheldon has formed an opposing team since he is so condescending to her. When Sheldon realizes she is on their team, his confidence is visibly shaken. They win the bowl when Sheldon does not allow one of his partners to answer a question, which turned out to be right.
After Leonard and Penny decide not to pursue a relationship, Leslie approaches Leonard for a true relationship, despite her previous unwillingness. She allows him to have the "male role," despite continuing to dominate and manage their relationship, and goes a bit far when she questions Leonard on any faults and genetic defects he has, and talks about having children on the first date. Sheldon is perturbed by Leonard's attachment to Leslie and ponders why, of all the overrated physicists in all the labs in all the world, it had to be Leslie Winkle, paraphrasing Humphrey Bogart in Casablanca. Following several interruptions by Sheldon during a date she had with Leonard, she interprets the situation as passive-aggressive. Later, in an attempt to be gracious and support his friendship with Leonard as Penny advised, he seems accepting of Leslie's romantic involvement with him. Sheldon claims to be willing to overlook her faults, but starts an argument that effectively nullifies her romance with Leonard by stating that she is an arrogant sub-par scientist, who prefers loop quantum gravity to string theory. She expects Leonard to back her up, but when he sides with string theoretic validation, she dumps him, feeling they are not compatible. ("How will we raise the children?")
In the next episode she suggests Penny's recent addiction to an online game is due to a lack of a sex life. Sheldon agrees, as Penny had already admitted to having not had intimacy for a while, making it a rare occasion that he agrees with Leslie on anything. When Sheldon signs up to use the mainframe in Buckman 204 at Caltech, Dr. Winkle rips the sign-up sheet off the wall as he printed it himself and put his name down in every slot for the next six months. Sheldon maintains that he was only trying to ensure university resources are not being "squandered chasing subatomic wild geese." This particular dispute could only be resolved by meeting with the chairman of the Physics Department, Dr. Gablehauser. In "The Cooper-Nowitzki Theorem," she makes fun of Sheldon after he gives an insulting lecture, but is derailed and disgusted when a graduate student named Ramona Nowitzki shows admiration for Sheldon. She even insults Leslie for Sheldon at one point (Sheldon is horrible at making snarky comebacks). Annoyed, she leaves.
"The Cushion Saturation" sees Leslie beginning a "friends-with-benefits" relationship with Howard when they are trapped in a shack during a paintball game. She gives him funds for his work and invites him on a trip to the CERN Large Hadron Collider project in Geneva. It turns out to be just Leslie's way to control Howard. Although initially reluctant to be in this sort of relationship, Howard later happily accepts his role as a "sex toy/arm candy." While Leslie does not physically appear in "The Vegas Renormalization," she plays a major part in the story line when she calls off her "friends with benefits" relationship with Howard. Having developed genuine feelings for her, he becomes depressed and goes to Las Vegas with Leonard and Raj, where he sleeps with a prostitute to get over her.
Leslie is completely absent throughout season 3, only appearing in the final episode, "The Lunar Excitation." Leonard shows up at her door and suggests they sleep together again after Penny had slept with him after they had broken up. She responds "Let me think about it," before slamming the door in his face. He realizes she is not coming back and she never does.