Sheila Gallagher (previously Jackson) is an agoraphobic, Frank's main love interest and a bit 'nutty' about shoes. While it's difficult for her to even think about leaving the house, she invites Frank into her home with open arms to take advantage of her cooking, disability checks and bedroom hospitality. Her clown-obsessed husband Eddie (that still resides in their basement, but then commits suicide) isn't making things any easier for her while she tries to cope with her phobias, Frank and raising their daughter Karen. According to her would be ex husband Eddie, she takes pleasure in sexual activities and taking control, something in which Eddie found no pleasure.
In Season 2 Sheila throws a party for Karen. It is noted that she is not very fond of Frank's mother, Peg. She begins to ask Frank why his mother is there, Franks tells her that his mother wouldn't miss the free food and alcohol. Sheila says that she doesn't want Peg there but Frank counters that his mother would "slaughter" him if asked to leave. Sheila goes back to the party and begins to make a speech about Karen's coming of age and all the new friends she had made, indicating Lip. Sheila also unintentionally reveals some of the unusual details of her sex life with Eddie, something that amuses Kev and Veronica greatly.
Things soon take a turn for the worse however when Sheila and Peg begin arguing. Sheila makes it clear that Peg isn't welcome as she is a convicted felon. The argument heats up and Karen and Jody are forced to come between the women and attempt to get Sheila upstairs. Kevin and Veronica are greatly amused by the whole argument and Lip begins to smirk just a little, until Peg is made so angry by a rather personal insult from Sheila that she pulls out a gun. Tony attempts to grab it from her but while everyone is panicking, a shot hits the fire detector. Frank takes this opportunity to call the police on his mother though Fiona enlists Tony's help in defusing the situation.
Season 3, Sheila was getting tired from Karen's baby Hyme, who had been crying constantly had gotten a point where Sheila had put the baby on top of a trash can as she was taking out the trash just as the trash truck showed up. She went back in the house to rest on the couch, but forgot the baby. Jody came downstairs and asked where the baby was and she immediately ran for the back door and picked up the baby basket which was almost picked up by a trash trucks claw. Frank showed up near the end of the episode unexpectedly and she then decides to let Frank stay, given the fact that Frank was able to put the baby to sleep. Unbeknownst to Sheila, it is a method that Frank used on Debbie when she was a baby. Debbie had babysat Hyme before.
Sheila eventually gets bored with her sex life with Jody only cuddling and doing it missionary style, something that bores Sheila which explains her marriage with Eddie and why Eddie wanted to end it during season one. She likes to take control and Jody however, refuses. She asked Frank about having some intimacy with her again. Franks refuses, but agreed to speak with Jody. Jody eventually agreed and gives it a try at first but after Sheila opens a box of her sex toys he gets a bit freaked out. Frank tries to help him by quoting the words "this is love". Once settled Sheila makes her move.
She eventually becomes depressed about her sex life with Jody. Frank and Carl were gathering a few things Eddie use to use and Carl asked him about a man's penis inside of another man. As Sheila was coming down the stairs she seemed depressed and ask Franks if he could baby sit Hyme. Carl begins to look at some of Sheila's sex toys and she took them from Carl and put tried to put them away in a hurry when a Nun in a wheelchair is brought to her home by the Father, to whom the Nun is taking a vow of silence. Sheila is instructed on how to take care of her and that the Nun likes her iPad charged all the time. Sheila reveals some of her intimate life details. The vow of silence however, did not coincide with the iPad the Nun had, as Sheila found out later that the Nun was blogging about her life. In the ending clip Sheila brings the Nun to a church and leaves her there. She then throws the iPad away in front on on oncoming vehicle.
Jody's sexual addiction begins to show again and around this time, she's enlisted by Frank to pose as Monica when his kids are taken away.
Sheila gets a little disturbed when Frank shows up at her house, which implies that her relationship with him is over. She forgets sometimes that Frank has the keys to the house.
After Karen's accident, she leaves for Arizona and Sheila is left alone. Sheila tries to cope with the fact that she's alone now and has no one. Debbie shows up after Karen leaves, Sheila asks if she could help with Lip's party and Debbie agrees and allows Sheila to accompany her. Close to the Party ending Debbie sits down with Sheila and tells her that she gets her loneliness, but that she needs to have friends in her own age group. It is possible that Sheila thinks Debbie is a replacement daughter while Karen is away. We then see Sheila at the end of the episode doing a group session of women looking to buy sexual products.
In season 4, Sheila starts internet dating, meeting an Native American, whose 5 little cousins she eventually wants to adopt. In order to do this, she must be married, so she tricks Franks into marrying her when he is in the hospital and thus begins calling herself Sheila Gallagher, instead of Jackson. Her request for adoption is still denied, however, and the children must live with their great-grandfather.
Just after Frank's opperation, he and Sheila are living together at her house. She's been caring for Frank and making sure he takes his all of his medications. While Frank works on his "Milk of the Gods" beer in the basement, Sheila contemplates an offer made to her regarding her house. Two women interested in gentrifying the neighborhood tell Sheila they would like to purchase her house for double the market value— and while Sheila desperately wants to accept and use the money to travel the world, but Frank disagrees. While the two argue over whether or not to sell, Sheila's anger toward Sammy (Frank's needy new daughter) grows. Sammy's attempts to get under Sheila's skin get more desperate, and Sheila's desire to escape intensifies. Frank tries several times to convince Sheila to stay, eventually asking her to keep her house so it can be "their nest". He blames his post-transplant weakness and desire to renew his and Sheila's vows for his resistance to selling. Sheila finally decides to cancel her plans and stay with Frank, until she, Sammy, and Frank get into a huge fight regarding the man Frank promised sex with Sammy. Frank blows up and screams at Sammy for being needy and annoying, and proceeds to turn on Sheila and accuse her of being a "certified wack-job" and a "pervert". Then, due to Frank's dangerous beer production in the basement, Sheila's house blows up. Ignoring the flames and the burning contents of her home, Sheila walks deliberately past Frank and Sammy and into the RV she bought earlier that day. Then, she is seen driving off as Frank makes one last-ditch attempt the get Sheila to stay. However, she decides to continue on and fulfill her dream of seeing the world.
Sheila is absent for the rest of the show.