As a child, she grew up with her brother Neil Morton who she cared for since he suffered a head injury. One night, her father came home drunk and attacked Sierra's mother before killing her. Sierra was told to not say a thing as he buried her mother in the backyard. Sierra's father was arrested and she was tended to by other relatives. She later grew up and became the mother of Lucas and had to raise him alone after his father became an addict.
She quickly starts a friendship with Fiona in Swipe, Fuck, Leave, while taking an interest in Fiona's brother Lip, even inviting them to the club. She immediately make her attraction known and they start a fling, though she states that she isn't looking for anything serious. Mostly because she has a young son (Lucas) that she is raising with drug-addicted, ex-boyfriend, Charlie.
At first, it was a casual sex but she shows comfort around him as she female ejaculates on him. This makes Lip both fascinated and scared about things getting too serious. Sierra asks him to take things to the next level and become Lucas's dad, but Lip wants to keep it casual. She takes offense to his rejection and has him leave, though they soon reconcile.
Sierra is motivated to end things with Charlie after he goes back to his habits and leaves Lucas alone with a friend. She also starts a real relationship with Lip, since he shows care for her and is there to comfort her in her moment of pain. However, she ends her relationship with Lip when he starts drinking again and attacks Charlie, as she couldn't bear the thought of dealing with two addicts.
After getting her own apartment in Ouroboros, she starts to reconcile with Lip when stating she met his mother who returned and finds she is a mess. She finds that he had a life similar to hers but she rejects Lip when seeing he is still continuing to drink while telling him to get proper help.
In the days after, she shows signs of trying to mend the fence between her and Lip by comforting Lip when his mother dies. Soon, she tells him that she will go with him to Monica's funeral and he thanks her. At the funeral, Sierra watches as Frank speaks up for Monica by making a touching statement and leaving. Following the funeral, she talks with Lip and decides to forgive him. They talk more as he tells her that he has decided return to college, even though he feels too old.
In "The Fugees", Charlie's ex has his baby, as Sierra is extremely upset when Charlie tells the truth and she leaves him in a huff. Lip shows up at Sierra's door, at the persuasion of Charlie. Sierra is shown in tears as Lip starts to explain that Charlie is crazy about her, but then she pulls him in for a kiss, which leads to sex. After they hook-up, Sierra asks him to stay, but Lip insists he should leave and they reconcile.
Sierra continues to look to Lip for emotional support. When her father (who went to prison for murdering her mother) makes parole, Lip stages a fight that lands her father back in jail. She sleeps over at Lip's house and says she loves him; Lip says it back.
Yet in the season finale, "Sleepwalking," he admits to Sierra that he isn't sure how to love her (even though he wants to). He urges Sierra to give Charlie a second chance. Sierra protests, but Lip insists it isn't her fault: he's been "sleepwalking through [his] life" and "loaded" since he was 12, and he isn't sure who he is or what he wants. He wonders if he can truly love Sierra when he was drunk, during most of their relationship. Sierra is devastated and tells him to go.
- She is Lip's only normal partner.
- She is one of the few women to show love to Lip.