Alexandra Claire 'Lexi' King (neé Nicholls) first appears hanging around Home Farm. Rodney Blackstock asks her to leave but she tells him she is related to the family. While he goes to check, she climbs into the house via the kitchen window, and when she is found, Carrie Nicholls says Lexi is her younger sister. Lexi stays and Carrie insists she get a job.
Lexi soon starts at the Woolpack as a barmaid. While Carrie and Lexi's relationship is difficult, Lexi is close to Carrie's daughter, Scarlett, and makes friends with Chas Dingle. Eli Dingle discovers that Lexi had been in prison in Mexico. After leaving the Woolpack, she gets a job at Eric Pollard's factory, and in late 2008, is appointed supervisor, over many more experienced and long-serving employees, causing resentment from other workers but they come round eventually.
In September 2007, Lexi, Chas and Debbie Dingle steal a necklace worth £50,000 from a jewellery auction at Home Farm. They plan to get Eric Pollard to sell it and split the money four ways. However, Debbie and Eric have a new plan: to tell Chas and Lexi that the necklace was a fake and quietly sell it, splitting the money equally. However, Lexi hears Debbie and Pollard discussing it and during a fight, the necklace goes down a drain. Horrified, Carrie asks Lexi what has happened to her since their mother died and Lexi says that she knows Carrie is her birth mother. She was born when Carrie was 14 and her parents adopted baby Lexi. Lexi wanted Carrie to acknowledge that they were mother and daughter but she refused. She did, however, tell Lexi that her father's name was Barry and he was 3 years older than her. Carrie was impressed because he had a car but wasn't in love with him. Disappointed, Lexi leaves the village for a while.
On her return, Lexi starts sleeping with Ross Kirk, unaware that he is also sleeping with Chas. Carl realises and tells Lexi who, furious, tells Chas. Chas sees that Lexi is interested in Carl King and warns her to stay away from him but Lexi ignores her and sleeps with Carl, in the hope he would make her Scarlett's trustee at King & Sons. Matthew becomes trustee and when Chas finds out about Lexi and Carl, she wants nothing more to do with them.
When Carl goes to work with Donald De Souza, Matthew King blackmails Lexi to spy on Carl; threatening to tell Scarlett that she had worked as a lap dancer if she refused. Chas realises what Lexi is planning and warns Carl but he hires her anyway. They soon get back together and as she falls in love with Carl, Lexi is increasingly reluctant to work for Matthew. Carl discovers Matthew using the proposals he was working on and blames Jimmy King. After punching Jimmy, he and Donald tell Carl the truth. Carl makes up with his brothers and Lexi loses her job. Although Carl is angry, they soon made up.
When Lexi returns from Carrie's wedding in Thailand, she tells Carl that Carrie told her that he murdered his father Tom King. The revelation makes Carl feel he has to stay with Lexi, instead of ending things with her for Anna De Souza. By December 2008, Lexi is involved with many King family activities and is living with them. After Matthew's death and the company going bust, they move into Mill Cottage. These changes make the family want to make a new start, and Scarlett wants to join her mother in Canada but Lexi persuades them to stay until the New Year, and later decide not to leave the area.
After a brief engagement, Lexi and Carl marry on 31 March 2009. The wedding almost does not happen, when Carl does not appear. Unknown to Lexi, he had been trying to persuade his former love, Chas, to be with him again. She refuses, so he goes ahead with the wedding. In April, Lexi tells Carl that she wants a baby and he unwillingly agrees. In June, Lexi discovers that she is pregnant but only days later, discovers her pregnancy is ectopic. A few days later, she learns that her remaining fallopian tube is badly damaged and is unlikely to conceive naturally. He recommended she and Carl try IVF treatment. Around the same time, Lexi lost her job when the factory closed. Carl refuses to go ahead with the IVF, finally admitting that he does not want more children. He suggests they separate if she wants children that badly but she decides Carl means more to her than a family of her own. However, on 13 August, Lexi and Carl finally confront each other on the hospital roof days after Nicola gave birth to her and Jimmy's daughter, Angelica. Carl admits he never loved her and only married her because she knew he killed his father. She then decides to leave the village.
"I'm managing, thanks." (First line to Rodney Blackstock)
"You know, I was gonna leave this for a bit, after what's happened. After how great you've been, but you stand there and blithely talk about dying. How do you think I'd feel if you died? If you die... If you die... Can't you even say it for me? You're such a bitch, you're such an evil bitch for making me say it. If you died I'd lose two mothers." (to Carrie Nicholls, revealing that Carrie is really her mother)
"You're a liar. And a cheat. Why the hell would I want to stay with you? We could've had such a wonderful life. Nevermind, eh?" (final lines to Carl King)