Alexandra "Lexie" Caroline Grey was the daughter of Susan and Thatcher Grey. She was a surgical resident at Seattle Grace Mercy West Hospital until she died in the plane crash, with her soulmate Mark Sloan by her side. Also known as Little Grey.

She had a younger sister, Molly Grey-Thompson. She had also an older half-sister Meredith Grey, born to Thatcher and his ex-wife Ellis. As a child she was bright and she had skipped third grade. She was very popular in high school, having a great group of friends, being crowned prom queen, and being class valedictorian. Near the end of Meredith's intern year, Thatcher mentioned to Meredith that Lexie was in med school and that she soon had to choose a residency spot.

By the time of Susan's death, both Lexie and Molly knew about Meredith's existence, though Lexie had never met Meredith before. On the day of her mother's funeral, Lexie waited in the car while Thatcher, accompanied by Molly, went to tell Meredith that she wasn't welcome at Susan's funeral, again missing the opportunity to see her older half-sister. After the funeral, Lexie went Joe's bar where she offered to buy Derek a drink. He was visibly charmed and called her forward, to which she replied that she had a bad day, so she got to be forward today. He confessed having had a bad day too, but decided not to have a drink with her because he was there with some friends. They both agreed it was too bad, showing interest in one another.

When Lexie found out that she had a sister through her father, she began to have fantasies of getting along with her. Soon, she went to Seattle Grace for her residency in hopes of meeting her. While Meredith did not appreciate Lexie's presence at first, over time, Meredith eventually tolerated her and accepted her as a sister. Lexie also confronted Meredith about her mistreatment of her on quite a few occasions, all with the good intentions of letting Meredith get to know her. After a very long time, Meredith began to care about her.

Lexie came to Seattle for her mother's funeral and after, she went to Joe's. There, she met her future brother-in law Derek Shepherd who she later discovered worked at Seattle Grace Hospital. Derek took a shine to her to the great annoyance of Meredith but more so, Cristina.

Voted student body president, Lexie was popular by both peers and teachers at school. Her photographic memory also made her one of the brightest in her class and not only made her a natural in the OR but also meant she knew all the periodic elements which she tells Mark Sloan at the beginning of season 5 when having a drink at Joe's bar.

When on Derek's service, she is left to operate with Meredith on a young woman's brain tumor, and with Meredith's go ahead, the tumor is removed. Both Big Grey and Little Grey believe they have successfully saved the young woman until Derek learns that they removed the tumor and shouts at them, explaining they weren't ready and they could have caused serious damage to the women's brain. When the women wakes up, her speech is slurred due to the surgery and her parents tell Derek they'll sue the hospital. Once Lexie hears about this, she breaks down and cries. It's in this episode we see her heart and how much she cares about the people she treats and how passionate she is about medicine.