She was portrayed by Laura Fraser.
She appears to be nervous most of the time and acts snobbish, ordering specialty teas in a diner. She played an unspecified role in Gus Fring's criminal empire, helping him distribute drugs throughout the American southwest. When the DEA begins investigating the link between Gus and Madrigal, Lydia becomes anxious and asks Mike to eliminate eleven people who were involved with Gus. When he refuses, Lydia makes the same offer to one of the eleven on her hit list and adds Mike to those she wants killed. However, Mike deduces her plan and dispatches the new hitman, but not before Chow is killed. Mike breaks into Lydia's home, ready to execute her, but allows her to live contingent on her providing him with methylamine.
After Lydia gives them the first shipment the DEA visits Madrigal and Lydia is forced to give up her guy in the warehouse, Ron Forenall, to the DEA. Lydia calls Mike to tell him she no longer has a guy in the warehouse who can get the methylamine and Mike assures her he will get her a new guy. Mike sends Jesse to Houston to pick up the methylamine, and he and Lydia find a GPS tracker at the bottom of one of the barrels. Mike later deduces that the DEA is not responsible for the GPS tracker and that Lydia is, because she wants out of the business. Mike vows to kill her again and mentions that operations may have to lay low for a bit without her as a source for the methylamine. Jesse, not wanting her to die, tries to intervene on the pretense that maybe the DEA really did plant the tracker. Looking to Walt to break the tie about the decision to kill Lydia, Walt sides with Jesse but only because he refuses to cease making meth. The three give her one last chance to prove she did not plant the device. After bugging Hank's office, they force her to call him and ask about the device. Hank tells her his office did not plant any trackers, and the three are about to kill her until they hear Hank, through the bug, call the Houston DEA office. The contact there tells Hank they placed trackers on every barrel, preventing the three from using any of them to make meth. In an effort to save herself, Lydia provides Walt, Jesse, and Mike information about a freight train that carries "an ocean of methylamine", thus giving them a new source of the chemical. Later, in order to make herself useful and therefore make Walt less eager to kill her, she provides Walt with a new market for Blue Sky in the Czech Republic, using her resources and network to distribute the meth in large quantities.
After Walt quits, Lydia inherits the meth business. One day, she shows up at the car wash and pleads for Walt to return to cooking, confiding her meth product has become less chemically pure without his expertise, but Walt refuses. Then Skyler coldly dismisses Lydia, warning her to never come back, much to Lydia's shock. She later meets with Declan and his crew in the desert, tours their underground meth lab, and unsuccessfully persuades an uninterested Declan (who is unconcerned that his subpar quality product jeopardizes her $50 million deal with the Czechs) to hire Todd as his cook. When Declan refuses, she orders Todd and his uncle's crew to massacre Declan and his men.
At the end of the series, Lydia and was poisoned with ricin and died.