WARNING: SPOILER ALERT
Fiona Gallagher (Emmy Rossum), age 26, is the eldest of the Gallagher children. Because of her mother's absence and Frank's neglect, most of the responsibility for child-rearing falls on her shoulders which forces her out of high school in her junior year. Fiona works a few dead-end minimum-wage jobs to support her five siblings. Though often stressed and exhausted by her responsibilities, she always performs them. Prone to selflessness, she sometimes needs to be reminded to look out for her own happiness. Even though Fiona wants everything under control, she enjoys a little bit of chaos once in a while.
At the close of season 2, Fiona receives her GED and begins to take more interest in jobs. In season 3, after Frank calls social services out of sheer spite, Fiona wins a court case (and with the admiration of the judge) is appointed her siblings' legal guardian. This strains her relationship with Jimmy.
Towards the end of season 3, Fiona finally gets a more rewarding job with a cup manufacturing company. In season 4, she begins dating her boss Mike but then has sex with Mike's addict brother Robbie. When Mike finds out he breaks up with her. In the aftermath of the breakup, Liam ingests cocaine that Fiona got from Robbie (a birthday gift; she kept the coke but made Robbie leave) at her birthday party and is hospitalized. Fiona is arrested for child neglect and drug possession. Mike posts her bail and she is put under house arrest with a curfew. When Fiona breaks her probation while partying with Robbie, she is incarcerated. She is released shortly afterwards though, due to overcrowding in the prison and gets a job at a diner.
In season 5, episode 4, Fiona impulsively marries Gus Pfender, the bassist of an indie rock band. She then cheats on Gus with Jimmy when he appears in town once again. Afterwards she begins a relationship with her boss Sean. Her extramarital affair with Jimmy, along with her budding relationship with her boss, Sean, results in her marriage ending. She discovers she is pregnant around the same time as Debbie but doesn't know whether the father is Gus or Jimmy, she has an abortion and urged Debbie to do the same. By the end of season 6, Fiona and Sean plan to be married, but Frank ruins the nuptials by revealing Sean's relapse to Fiona at the altar, causing Sean to leave her.
By season 7, Fiona starts to act more selfishly as she is tired of being unappreciated by her family members, especially Lip, and she is more focused than ever on building wealth. Her boss Margo is also a high school dropout but has amassed a net worth of $300 million; Fiona wants to follow in her foot steps. She sees an opportunity to invest in a deteriorating laundromat and puts the Gallagher home as collateral which angers Lip. She eventually sells the laundromat to Margo. She then buys an apartment building.
When Monica dies, Fiona is indifferent, declaring her willingness to have the funeral consist of a bonfire in the funeral home parking lot with the ashes stored in a bag. Although it doesn't come to that, she does bury her portion of the inheritance in Monica's casket.source