Andrea Cantillo was the mother of Brock Cantillo and a recovering addict.
Jesse Pinkman met Andrea in a Narcotics Anonymous (led by the Group Leader) while he was trying to push the blue meth he stole from Gustavo Fring's superlab. Andrea was the single mother of a six year-old boy, Brock Cantillo. She also had a 10 year-old brother Tomás Cantillo, though the two were estranged. Andrea later revealed unknowingly to Jesse that Tomás killed Combo ("Abiquiú"). A couple of days later, after a series of events involving the Rival Dealers, Tomás was found shot dead ("Half Measures").
After finding out about Tomás' death, she received a handful of cash in her mailbox from Jesse, who wanted her to use the money to move to a better neighborhood instead of buying drugs ("Thirty-Eight Snub"). She did and found a modest home for herself and Brock in a nice neighborhood ("Hermanos").
She was again seen, with Brock, at Jesse's House shortly after Jesse returned from Mexico. Walt interrupted their evening together to plead for his life ("Crawl Space"). Later, Andrea took Brock to the hospital after he suddenly becomes ill from some unknown cause. Jesse warned her that it might be ricin poisoning, however tests revealed he was actually poisoned by the berries from a plant called Lily of the Valley ("End Times") ("Face Off").
Jesse, Andrea and Brock were happy in their relationship until Walt made Jesse think about the time in the future when Jesse would have to confess to Andrea his involvement in the meth trade. In response, Jesse broke things off with Andrea ("Hazard Pay").
While Jesse worked with Hank to bring justice to Walt, Walt visited Andrea. He told her that he and Jesse got into an argument and Jesse could not be found. He also mentioned that Jesse had been using drugs again. In order to help Walt out, Andrea left Jesse a voicemail, telling him to call her back later. During this visit, Walt also tried to make conversation with Brock, who answered his questions in a curt manner. ("To'hajiilee")
After Jesse tried to escape from Jack Welker and his gang, Andrea was killed by Todd Alquist as punishment to Jesse for attempting to escape and refusing to cook for him ("Granite State"). Jesse later avenged her death by strangling Todd to death with his handcuff chains after Walt massacres Jack's gang. ("Felina")