Dr. Lexie Grey
Background
Lexie is the daughter of Thatcher Grey and his second wife, Susan. She also has a younger sister, Molly Grey Thompson. Her older half-sister is Meredith Grey, born to Thatcher and his first wife, Ellis Grey. She graduated from Harvard Medical School before being accepted in Seattle Grace's Intern Program. She was very popular in high school, having a great group of friends, being crowned prom queen, and being class valedictorian. She has a photographic memory, which was highlighted in the fifth season when she remembered an illness she came across while reading medical journals for George's review. She not only remembered the diagnosis but the name of the journal, the volume number and the page number of the article.
As seen in the previous season five episodes, she is the only intern who can stand up and speak her mind to the other residents, possibly because of her relationship with Meredith Grey. She objected when Cristina Yang made them line up in the cafeteria to discuss which interns to pass on to George (who was now a resident) citing that they were treated like "livestock." When Alex Karev took over the several cadavers Lexie had initially put out for the other interns, she also objects. However, she is seen to eventually acquiesce to the residents' orders.
Season 2
Lexie is mentioned in the episode in which her sister Molly is pregnant and taken to Seattle Grace to have a surgery to help the baby breathe. When Meredith is talking to Molly about Meredith's paternal side of the family, she mentions her sister and her education at Harvard University.
Season 3
She was introduced in the penultimate episode of the third season in which she met Derek Shepherd at Joe's Bar. Her mother Susan suddenly died when Lexie was just about to start her internship at Mass Gen. In the third season finale, Derek confesses to Meredith he met another woman (though he doesn't specify who, since he doesn't know) and when she asks if she should be worried, he confirms her fears. Lexie would make her first appearance in Seattle Grace when she introduced herself to George O'Malley, who realized her connection to Meredith as the episode ended.
Season 4
Lexie is assigned to surgical resident Cristina Yang for her year-long medical internship, and bears striking similarities to her half sister, Meredith Grey. She tries on several occasions to bond with Meredith, who wants nothing to do with her. Meredith sees Lexie's life as the one she could have had if her father hadn't left her and her mother, and thus has no desire to bond with her initially. Later, Meredith sits down with Lexie and explains the circumstances around her mother's death when Meredith was caring for her, suggesting they might kindle friendship.
Lexie learns that George O'Malley is repeating his intern year and helps him keep the other interns unaware of his failure. She and George develop an unlikely friendship over intern struggles and their strained relationships with Meredith and Callie, respectively.
She has entered into a casual-sex relationship with Alex Karev, similar to the one between Meredith and Derek Shepherd. Lexie was angry when she learned that Alex still had feelings for another woman who believed they were still a couple. She bonds with a patient named Nick, played by Seth Green, who admits he likes Lexie; however, he dies in Lexie's care and she is devastated. Meredith allows her to stay over the following night and decides to try and repair their relationship, making Lexie breakfast the next morning (unaware that she is allergic to eggs and causing her a massive breakout of hives.) She and Meredith have a good relationship after that, with her referring to Meredith as her sister, and constantly apologizing for Rose's actions. George moves in with her after Izzie kicks him out, and she steals items from the hospital to furnish their ugly apartment. At the end of season four, Lexie and George share a quick kiss on the lips. It is uncertain if this a friendship kiss or hints to a romantic relationship in the fall, though Lexie's expression makes it clear her interest falls to the latter.
Season 5
In season five, Lexie is more obviously smitten with George, saying, "I adore you" out loud once he leaves the room. Mark Sloan notices this and vows to be less mean to George if she'll admit her feelings to him. As a result, Sloan is slightly more polite to the two of them and George misunderstands, thinking they are sleeping together. When Lexie excitedly asks him if he's jealous, he seems nonchalant. She goes out on a limb and tells George she believes they are more than just roommates; much to her dismay, he responds that of course they are — they're best friends.
Lexie continues to vie for George's affections in the following episodes. She correctly diagnoses a patient who had been experiencing constant pain at an eight for several years, much to Cristina's chagrin and her colleagues' amazement, and the consequent revealing of her photographic memory caused her to be nicknamed Lexipedia. She sat by George when he opened the letter regarding his residency, and he ignores her when she offers to buy him a drink at Joe's to celebrate. When she arrives at the bar, she finds that George was already celebrating with Meredith, Izzie and Cristina, much to her dismay.
George finally passes his intern exam and seems to treat Lexie with even less respect. The final straw occurs when he receives his new interns and Lexie discovers he hadn't the decency to even ask to have her assigned to him. She breaks down in front of him, calling herself stupid and him, a jerk.
By the next episode Lexie had chosen to forgive George and offered him a peace offering; leaving her feelings for him undiscussed.
She, with the other interns, form an underground 'teaching' club, where they learn to put IV's on each other due to the fact that their residents don't bother to teach them. A new intern, Sadie, who happens to be friends with Meredith comes to Seattle Grace. After Meredith asks if Lexie is a secret cutter, she begins to think that they should be more careful but Sadie says they will "learn only so much by being careful" and cuts her back and offers to let an intern stitch her up. Lexie, watches disapprovingly. After receiving praise from the Chief of Surgery on the quality of her suturing, Dr Yang grows suspicious and orders the interns to stop practicing on themselves any medial procedures.
Lexie is the only one aside from Callie, Meredith, and Derek who sees right through Mark Sloan and isn't afraid to tell him the truth. He visits her at Joe's once and asks her about her photographic memory, and she impresses him with what she can do. Their platonic relationship soon turns romantic, however, after she becomes impressed when he fixes a woman's vocal chords. She shows up at his room later and the two sleep together. In the episode after that, Mark tells her it was a "one-time thing" but to Callie, he reveals it was more than just sex because apparently he and Lexie talked and laughed for hours after they slept together. Though he and Lexie make an agreement not to give in anymore, Mark realizes he can't deny his attraction, and asks Lexie to come home with him again. Their sexual relationship continues until during the midst of another sexual encounter, Lexie accidentally injures Mark's penis. While Mark receives surgery to correct the injury, he remains embarrassed and tried to push Lexie away, although she refused and instead chose to comfort him.
Later, when he and Derek are decorating Derek and Meredith's room with flowers for Derek's proposal, Mark talks to him about love and the future, and Mark seems to be very serious about Lexie and their relationship. Lexie later breaks up with Mark when he doesn't want to come forward with them as a couple. When Mark and Derek's friends Addison, Naomi Bennett and Sam Bennett come to Seattle Grace, Addison soon notices the tension between Lexie and Mark, and wonders if they are together. Later, Lexie hears about a fight Mark had with Derek, and understands that it was partly because Mark told Derek about his relationship with her. When she arrived at his hospital room where Callie was treating him, she found Mark beaten up, his hand injured. Lexie told him he was being stupid, but Mark said it was worth it, and smiled proudly when both realized people would soon know about them.
In the next episode "I will follow you into the Dark", Lexie tries to get Mark to talk to Derek, but Mark, who still has his injuries and can't operate yet, refuses to do so and walks away angry. Later Lexie goes to him and tells him that she knows everybody is against them and their chance of survival is very little, but she wants to try it anyway. She also tells him she's sorry for breaking his hand, his penis and just him. Mark then tells she didn't break him, she's the one who put him back together, and then they walk out together.
There is a strong possiblity that she and Mark could be one of the suspected couples to get married in the 100th episode.
Source: Wikipedia.org