Stephanie makes her debut in "The Lizard-Spock Expansion" (S02E08) where Howard Wolowitz tries to seduce her by taking her to a Jet Propulsion Laboratory and lets her drive a Mars rover. When he gets the robotic car stuck in a ditch, he calls his friends over for help and asks Leonard to drive Stephanie home. This act proves to be lethal for Howard's chances with Stephanie, as she and Leonard end up making out in the car. She eventually confesses to Howard that she's interested in Leonard and not him, which Howard takes as Leonard's betrayal of his trust. Howard starts acting like Leonard is dead to him; however, he forgives Leonard and Stephanie, when Stephanie sets him up with her roommate Lisa.
Stephanie continues dating Leonard in "The White Asparagus Triangulation" (S02E09), which leads to several encounters between her and Sheldon Cooper. Although she seems to put up with Sheldon's obnoxiousness, she uses her cunning to keep him under control. For instance, she makes him believe he has to stop talking because of his inflammed larynx in "The Vartabedian Conundrum" (S02E10), which keeps Sheldon mute until the end of the episode.
By "The Vartabedian Conundrum" (S02E10), she has established herself in Leonard's apartment, which leads to Sheldon and Penny pointing out that he and Stephanie are living together. Leonard finds that his closet is full of her clothes and her jewelry box has replaced his Bat-signal on his dresser. He realizes that the relationship is moving too quickly for him, so he insists that she relocate to her own apartment. He does have trouble shaking her off, since when he tries to talk to her, she makes sexual advances on him and the two end up having sex. Stephanie seems to be insecure in her relationships and counters by being very aggressive sexually. She is also uncomfortable with Penny wandering into the apartment in her underwear without even knowing who she is. When Leonard attempts to ask Stephanie to move out, she assumes that the relationship is heading to its imminent end, but Leonard quickly backtracks. Howard suggests that Leonard ask her to move out via text. Leonard does so, and regretfully states that he will never have sex again. His phone buzzes, he looks at it, says he was wrong, and promptly leaves.
Although it's not seen on screen, it is known that some time between "The Vartabedian Conundrum" and the following episode "The Bath Item Gift Hypothesis" (S02E11), she and Leonard broke up, but the details about their break up are unknown. It is also unknown what has become of her and if she and Leonard continue to be friends. In "The Valentino Submergence" (S09E15), Raj asks Howard and Leonard if they have ever broken up with anyone, and Leonard says, "Not really."
Stephanie at first met Howard in a bar, and went with him to the Mars Rover lab to drive the rover; however, Howard got the Mars rover stuck in a ditch and he asks Leonard to take her home. She and Leonard end up together. When she breaks up with Howard over the phone, Howard is infuriated and sees her and Leonard dead to him. She later sets Howard up with her roommate Lisa and both she and Leonard are forgiven.
- Main article: Leonard and Stephanie
Stephanie's primary role in the show is being Leonard's girlfriend. They met in "The Lizard-Spock Expansion", and continue dating for two additional episodes. Eventually it seems they broke up, but it is unknown how and when though Leonard thought their relationship was moving too fast after he realized that she had moved in with him.
- S02E08: The Lizard-Spock Expansion
- S02E09: The White Asparagus Triangulation
- S02E10: The Vartabedian Conundrum
- Stephanie did her medical internship at Lawrence Memorial (fictional) in Galveston, Texas, which is where Sheldon was born though Mary Cooper said that she dropped him at the K-Mart.
- Stephanie is the first of Leonard's girlfriends who Sheldon really liked. Later, Sheldon admits that he likes Penny.
- Stephanie was mentioned only one time since her breakup with Leonard in "The Apology Insufficiency", when Sheldon mentions that Howard got the Mars rover stuck to impress her. It should also be noted that Sheldon didn't even refer to her by name.
- Out of all of Leonard's exes (Joyce Kim, Priya Koothrappali, Leslie Winkle and Penny), she is mentioned the fewest amount of times by the cast.
- When Sheldon describes their group as a Star Trek landing party, he is Spock, Leonard is Kirk, Howard is Scotty, Raj is the guy in the red shirt that is killed and the newest member Doctor Stephanie is Dr. McCoy.
- Stephanie Barnett on IMDb