<P> The night before her internship begins, Lexie meets Derek Shepherd (Patrick Dempsey) at Joe's Bar, mirroring his initial meeting with Meredith . The pair flirt, but Derek makes his excuses and leaves . On her arrival at the hospital, Lexie meets George O'Malley (T.R. Knight) who quickly realizes her identity . Lexie promises to keep George's secret that he is repeating his internship after failing his intern exam and the pair strike up a friendship . Lexie is assigned to be Cristina Yang's (Sandra Oh) intern, and is dubbed "three" by Cristina, who doesn't take the time to learn her name . Lexie is eager to get to know her half - sister Meredith, but receives a hostile response when she introduces herself, and her later frequent attempts to bond with Meredith are similarly rebuffed . Lexie begins a brief sexual relationship with Alex Karev (Justin Chambers), who discovers that Thatcher has descended into alcoholism following Susan's death . When Meredith scolds Lexie for not looking after Thatcher, she finally retaliates and decides to stop pursuing a relationship with Meredith . Lexie bonds with patient Nick Hanscom (Seth Green), and is present when his exposed artery blows, causing him to suffer massive blood loss . She manages to stop the bleeding, but is distraught when Nick later dies anyway . Feeling sympathy for Lexie, Cristina invites her to join her and Meredith in drinking and dancing, causing the sisters relationship to begin to thaw . The following morning, Meredith goes out of her way to make Lexie breakfast, which she politely eats despite an allergy to eggs, resulting in her having to be treated at the hospital . Lexie and George agree to move in together, but they can only afford a dilapidated apartment which Lexie attempts to improve by stealing decorations from the hospital . She begins to develop romantic feelings for George, and discovers that he had only failed his intern exam by one point, encouraging him to try and convince the Chief that he deserves a second chance . </P> <P> Lexie continues to harbor romantic feelings for George, oblivious to the fact that he doesn't see her in the same way . Their relationship is a mirror foil of George's previous infatuation with the oblivious Meredith . She prioritizes helping George study over taking part in a surgery with Mark Sloan (Eric Dane), but feels betrayed when George doesn't request that she become one of his interns after passing his exam . Finally realizing that he doesn't feel the same way, Lexie gives up on her feelings for George and the pair's friendship begins to fizzle out . She later begins a flirtation with Mark, and the pair begin an unlikely romantic relationship, though they are forced to keep it a secret when Meredith and Derek warn Mark away from Lexie . Lexie discovers that some of her fellow interns have secretly been performing simple procedures on each other and begins taking part to prove she's hardcore . Sadie Harris (Melissa George) joins the society, and seeking a more daring procedure, suggests removing her appendix . Though Lexie agrees, she quickly finds herself out of her depth, resulting in Meredith and Cristina having to intervene to save Sadie's life . Lexie and the other interns are put on probation . Derek finds Lexie distraught at the day's events, and allows her to move into the attic at his and Meredith's house . Mark comes clean to Derek about his relationship with Lexie, resulting in the two men getting into a fist fight . The pair continue to feud, resulting in Lexie beginning to stress eat until they eventually reconcile . Lexie is delighted when Meredith asks her to be a bridesmaid at her and Derek's wedding, though they eventually give the ceremony to Alex and cancer - stricken Izzie Stevens (Katherine Heigl). Mark decides to purchase a house and invites Lexie to move in with him, however she declines, concerned about how fast their relationship is progressing . </P> <P> Lexie feels guilt following George's death, having abandoned their friendship after he failed to reciprocate her romantic feelings towards him . Mark comforts her, and she eventually agrees to move into his new apartment with him . Thatcher is admitted into the hospital with liver failure stemming from his former alcoholism . When Lexie is found to not be a suitable transplant candidate, Meredith steps in and donates part of her liver, not to help Thatcher, but to spare Lexie the grief of losing her father and because Lexie inappropriately pulled Meredith's medical files and begged her to . Mark and Lexie receive a shock when a pregnant teenage girl named Sloan Riley shows up claiming to be Mark's daughter, and her parentage is soon confirmed . Mark quickly agrees to let her and the baby move in permanently without consulting Lexie, resulting in her ending their relationship when Mark asks her not to force him to choose between Lexie and his child and grandchild, because he'd have to choose them instead of her . She engages in a one - night stand with Alex, but feels guilty and confesses to Mark when he attempts to restart their relationship once his daughter leaves . Mark is furious with her, officially ending their relationship . Despite secretly suppressing her feelings for Mark, Lexie and Alex assume a casual relationship, that Lexie begins to take too seriously and flaunts to make Mark jealous . Lexie is able to use her eiditic memory to help Alex with several of his cases . Meanwhile, Mark and Lexie never seem to get their timing right, with Lexie disinterested and angry at Mark when he wishes to reconcile, or Mark being caught sleeping with a flavor of the day when Lexie wants to rekindle their relationship . Lexie takes part in a surgery on a patient named Alison Clark (Caroline Williams) and informs her husband Gary Clark (Michael O'Neill) that it was a success . However, moments later Alison suffers a stroke and falls into a coma that she is deemed unlikely to wake from . As Alison had signed a DNR form, Lexie is forced to turn off the machine keeping her alive, despite Gary begging her to stop . After failing in an attempt to sue the hospital, a grief - stricken Mr. Clark later returns to the hospital with a gun, seeking revenge on Lexie, Derek and Richard Webber (James Pickens, Jr .). After witnessing Mr. Clark shoot a nurse dead, Mark shields Lexie during the shooting and the pair attempt to save a critically wounded Alex . Seeking supplies, Lexie heads out into the hospital and comes face to face with Mr Clark who attempts to shoot her, but she is saved by a SWAT team member who wounds Clark at the last moment allowing her to escape . Lexie and Mark proceed to save Alex's life, but Lexie is disappointed and irritated when a delirious Alex calls for his ex-wife, Izzie, while nearly dying instead of Lexie . After a showdown with Richard, Gary Clark eventually commits suicide, having killed eleven people and wounding another seven, as well as causing Meredith to suffer a miscarriage . </P> <P> Lexie has a psychotic breakdown after the shooting . She is admitted to the hospital's psychiatric facility and is sedated for over fifty hours, with Meredith remaining by her side . Alex, on the other hand, breaks off his and Lexie's fling because he cannot handle having to care for an insane person after his personal and family history . Lexie retaliates with causal cruelty by mocking him for calling out for the wife who left him and snubbing him when he asks her to use her "Lexipedia" talents to help on cases . Lexie is paranoid and irritated that other's may view her as fragile and incompetent due to her melt down and works hard to repair her image . After both realize they continue to have feelings for each other, Lexie and Mark resume their relationship, but their brief happiness is ruined when Lexie discovers that during their breakup, Mark had impregnated his best friend Callie Torres (Sara Ramirez) and she again ends their relationship . Eager to win Lexie back, Mark recruits his protégé Jackson Avery (Jesse Williams) to try to bring him back into Lexie's favour, but his plan backfires when Jackson and Lexie develop romantic feelings for each other, resulting in them starting a relationship of their own . Mark is furious and tries to win Lexie back, but shortly after Callie is involved in a car accident and Lexie consoles Mark . The next episode, Lexie explains that she will go back to Mark if he continues to pursue her, but she believes they ultimately aren't suited and will be unhappy . Mark agrees to let her go, giving her and Jackson his blessing . </P>

When does lexie grey come into the show