Diane Vause appears exclusively in Alex's flashbacks. She is a single mother who works four jobs to keep her and her daughter going.
After a young Alex threw her imitation sneakers out of the car window in response to being bullied about them, Diane pulls over the car and talks to Alex about the girls who are bullying her. She reiterates that the girls are nothing and that Alex's dad is a famous rock god ("Fucksgiving").
In "Tall Men with Feelings", flashbacks show an adult Alex receiving the news that her mother has died. She gets a call from her aunt who explains Diane died of an aneurysm. In this flashback, Piper is breaking up with Alex, and leaves even after Alex has received the news her mother has died, leaving Alex to deal with everything alone in a strange city ("Tall Men with Feelings").
In "Fear, and Other Smells", a flashback shows a devastated Alex at her mother's funeral. When Fahri picks her up afterwards, Alex expresses regret at the small number of people at the funeral and that she should have been there for her mother. Fahri soothes her by saying that she didn't know what was going to happen.
Diane's death, coupled with Alex's break up with Piper, causes Alex to break down and become increasingly dependent on drugs.