Dr. Leslie Winkle, Ph.D., is an experimental physicist at the California Institute of Technology who works in the same laboratory as Leonard. 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. 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. Leslie made her final appearance in "The Lunar Excitation" when she rejected Leonard's offer for casual sex and left the series after that, but then reappeared in "The Celebration Experimentation" in the ninth season to celebrate Sheldon's birthday.
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 ("The Bat Jar Conjecture"). 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. This is Leslie's last appearance in the show for six years.
Leslie has no appearances in any episodes of Season 4.
Leslie has no appearances in any episodes of Season 5.
Leslie has no appearances in any episodes of Season 6.
Leslie has no appearances in any episodes of Season 7.
Leslie has no appearances in any episodes of Season 8.
Season 9She appeared in the 200th ("The Celebration Experimentation") episode as a guest at Sheldon's birthday party. She speaks with Leonard, who reveals he and Penny are now married, and she comments she assumed he and Sheldon would always live together (not realizing they do, in fact, still live together). During her toast she acknowledges she would normally insult Sheldon by calling him 'dumbass,' but since they have matured she only wishes him a happy birthday. But as Sheldon does not like change, he gives her permission, and she happily declares, "Happy Birthday, Dumbass!" to which everyone cheers.
Leslie specializes in high energy physics, particularly searching for supersymmetry with like-sign dileptons. Leslie believes loop quantum gravity is the future of physics and better unites quantum mechanics with general relativity than string theory does. She posits that only loop quantum gravity calculates the entropy of black holes and expects quantized space-time to manifest itself as minute differences in the speed of light for different colors.
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.
By the ninth season she is shown to have matured so that she does not feel a need to insult Sheldon, but she does anyway since Sheldon insisted (he hates change).
Leonard tries to ask her out because he considers her to be in his league and they decide to kiss to see if they have sexual chemistry. Though Leslie compliments Leonard on his kissing technique, Leslie feels no arousal, and thus, no date occurs. She eventually puts forth an offer of sexual availability towards Leonard one evening after a string quartet practice in which she plays the violin and the two engage in sexual intercourse, but she reveals that she only desires sex infrequently while distracted by a difficult project and subsequently dumps him, claiming she'll be satisfied until at least New Year's Eve.
Initially uncomfortable with commitment, she later comes to the realization that "there’s just a time in every woman’s life when she gets tired of waking up on a strange futon with a bunch of people she doesn't know." Leslie briefly tries to rekindle her relationship with Leonard after his failed attempt at dating Penny, stating that she is "fully committed to the traditional relationship paradigm" and "who better to slow things down with than you?" She even considers Leonard as a potential mate, going so far as asking Leonard how many children they should have. However, it does not work out because of Leslie's belief in loop quantum gravity and Leonard's belief in string theory, which is, as she put it, a "deal-breaker." Leonard later said he did not care about their relationship. In "The Lunar Excitation", when Leonard appears at her door, Leslie reveals it has been eighteen months since they have seen each other. He asks if she wants to have sex with him just for fun like they used to. She quickly deduces that Penny dumped him and says she would like to think about it, before she slams the door. Leonard realizes she is not going to come back. Despite their history of not sustaining a relationship for any significant length of time, Leonard thoroughly enjoys watching Leslie spar with Sheldon, as she is one of the few people who can catch him off-guard.
In "The Celebration Experimentation", Leonard approaches Leslie and they agree that it's been a long time. Leonard says that things have changed but Leslie feels that they haven't. Leonard mentions having married Penny which Leslie remarks that she never thought he would move away from Sheldon. Leonard says "Yeah," since he and Penny still haven't moved away.
Sheldon originally had no animosity towards Leslie, as he was the one who encouraged Leonard to pursue her. After her first dalliance with Leonard, she was able to fix a problem Sheldon was having in showing that quarks are asymptotically free at high energies by changing the sign of the beta function β(g) of quantum chromodynamics (QCD) on his whiteboard, greatly irritating him. When she insulted his intelligence, he came to hate her, considering her his "archenemy" playing the role of a "Dr. Doom to [his] Mr. Fantastic." She even, at some point in the past, made Sheldon cry. Sheldon usually insults her, but had stated he has no respect for her as a scientist or a human being. Sheldon thinks her research methodology is sloppy, asserts that she’s unjustifiably arrogant about loop quantum gravity and that the only way she could make a contribution to science would be if they resume sending chimps into space. Sheldon also conceded that she had an undeniable expertise in the interrelated fields of promiscuity and general sluttiness. To make matters worse, according to Sheldon, she's often mean to him.
Meanwhile, Leslie Winkle considers Sheldon an "arrogant, misogynistic East-Texas doorknob" who one time told her she should abandon her work with high energy particles for laundry and child bearing. She has likewise grimaced that he is a "passive-aggressive East Texas blowhole." More often, Leslie displayed indifference to him, frequently calling him "dumb-ass" and insulting him at every opportunity. A commonly used insult (or a slight variation) by Leslie is "Hello, fellow scientists. Hello, Sheldon." Her favorite quip to him is "Hello, dumbass." When Sheldon received a phone call during a meeting with Dr. Gablehauser, she hoped he wouldn't miss a call from the Nobel committee informing he had been nominated as dumbass laureate of the year. Similarly, when Sheldon was working toward a breakthrough with his theory of string-network condensates at lunch in the Caltech cafeteria, she remarked that he was organizing his papers for the Smithsonian Museum of Dumbassery. The only time Sheldon got the better of Leslie was when he tricked her into breaking up with Leonard, and this action was primarily because Leslie was busy arguing with both of them rather than focusing on Sheldon.
Interestingly, during "The Cushion Saturation," neither displayed any animosity towards each other, suggesting they have gotten past it; however, this action was probably because Sheldon's temper was currently directed at Penny instead.
In "The Celebration Experimentation". Leslie toasts Sheldon that in the past she would have said something obnoxious like, "Happy Birthday, dumb ass". She feels she has grown, is just glad to see everyone and adds, "Happy Birthday, Sheldon." Sheldon doesn't really like change so she shouts it out to him anyway. "HAPPY BIRTHDAY, DUMB ASS!!"
In "The Cushion Saturation" (S02E16), during a Caltech paintball game, she begins an affair with Howard Wolowitz. When telling Leonard, Raj, and Sheldon of his relationship with Leslie, Howard reveals that Leslie is the fifth woman to sleep with Howard without being paid (*). She allows him access to experimental equipment (like a rapid prototyper) in spite of budget cuts and a research trip to Geneva to check out CERN's supercollider, using some unrestricted grant money Caltech gave her. She essentially controls Howard with new equipment and research trips, so as to avoid a real relationship with feelings, and institutes a "friends with benefits" arrangement instead, which she is more used to. However, in "The Vegas Renormalization", she dumps him over the phone, simply saying: "Howard, mama’s a rolling stone."
- Sheldon became angry when Leslie and Leonard briefly dated, when, ironically, he was the one who suggested Leonard pursue her in the first place. When he did; however, he had no ill feelings towards her.
- He showed no anger when she and Howard were together, possibly because they were not officially dating. Also, since Leonard is his best friend, he probably didn't mind what he did.
- Leslie's only appearance in the third season was its finale, which was also her last appearance in the series before the 200th episode in the ninth season.
- Out of all of Penny's rivals for Leonard's affection, Leslie was perhaps the smallest threat, as she and Leonard only dated for one episode. She is really more of a friend to the guys, since she is on their intellectual level and shares their quirkiness.
- Leslie is one of the only people who can make Sheldon stutter.
- On March 11, 2014, Mayim Bialik (Amy) visited "The Talk" and was interviewed by Sara Gilbert (Leslie Winkle). They both appeared on "The Lunar Excitation"; Amy's first appearance and Leslie's last appearance for 6 seasons. Both were main characters at one time.
- Though Leslie was promoted to main character status in the second season, she did not appear in many episodes. The reason is because the writers were not certain how to write in the character full time, leading to her return to recurring status and eventual departure.
- Sara Gilbert (Leslie Winkle) and Johnny Galecki (Leonard Hofstadter) played married couple Darlene and David on Roseanne (1988- 1997).
- Leslie may not have seen Leonard for eighteen months at the time of her last appearance; however, she did know that he had dated Penny.
- Leslie was briefly a regular on the show, as in the opening credits, Sara Gilbert's name was listed.
- Leslie was never shown to have met any of the other later additions to the cast: Bernadette Rostenkowski-Wolowitz, Amy Farrah Fowler, or Stuart Bloom, until "The Celebration Experimentation" in season 9, all of whom were at Sheldon's birthday party. She has presumably still not met Emily Sweeney.
- Between her last appearance in the Season 3 finale and her reappearance in Season 9, Leslie has missed six years, six seasons, and a span of 137 episodes.
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Green = Appeared,
M = Mentioned, V = Voiced, F = Flashback, C = Cameo
Dark Grey = Did not appear,