Selma Bouvier was born two minutes before Patty. Due to a childhood bottle rocket accident, she lost the senses of taste and smell. Selma's favorite film actor was reportedly Troy McClure, to whom she was briefly married before discovering it was just a publicity stunt by McClure. She was willing to live a sham for a while, but broke it off when she realized she couldn't bring a child into a loveless marriage. According to Marge, Selma is "the one who likes Police Academy movies, Hummel figurines, and walking through the park on clear autumn days."
Selma inherited her deceased aunt's iguana, Jub-Jub, instead of her mother, Jacqueline adopting him, due to Jacqueline's apparent dislike of Jub-Jub. Unlike Patty, Selma showed brief moments of sympathy for Homer, primarily due to his unwavering loyalty to his family. In fact, Selma is responsible for reuniting Homer with Marge after they split up, despite an agreement with Patty not to do so.
Homer also demonstrated moments of kindness towards Selma such as offering advice and sympathy, when she began to question her own ability to be a mother after attempting to take care of Bart and Lisa for an afternoon. Later, Homer was convinced by Marge to act as Selma's husband so she could adopt a Chinese baby and did all he could to make sure Selma got to keep said baby. He also helped her and Patty keep their jobs after they were caught smoking at work (although this was more for Marge's sake). However, these instances are still very few and far between. Unlike Patty, Selma is very desperate to find love after seeing her Aunt Gladys' tape. She also asks Marge for advice to find love, saying Patty is 2 minutes more beautiful than she is.
Selma's favorite brand of cigarettes is identified as "Laramie Hi-Tars" while Patty's favorite brand of cigarettes is "Lady Laramie 100s"; they have been claimed by their mother to "have started smoking even before birth". They share an apartment at the Spinster Arms apartment complex and work at the Department of Motor Vehicles (DMV). They on numerous occasions acted unprofessionally in the course of their jobs.
Selma lacks the senses of taste and smell due to a childhood bottle rocket accident (one shot up her nose). Both twin sisters tend to be cynical and are noted for their addiction to tobacco smoking. They have a strong, mutual (and reciprocated) dislike for their brother-in-law. She (along with her sister) are shown to be older than Homer and Marge, but a birthdate has not been given. It is presumed they are 41.
Unlike Homer, who usually makes a perfunctory (if not particularly sincere or committed) effort to be polite to his sisters-in-law out of respect for his wife, Patty and Selma have no hesitation in treating Homer with open rudeness and contempt. There were several instances where they did not seem to care when Homer faced a life-threatening situation. For instance, when Marge rushes to the hospital after receiving a telephone call Homer suffered a heart attack at work, Patty and Selma — instead of going to the hospital and comforting Marge in her time of need — remained seated at the kitchen table, continuing their activity of cutting coupons, gasping in amazement at a five cent discount and behaving as though nothing was wrong. They later attempted to set up Marge with a (rather sleazy) friend as a replacement for Homer, regardless of the fact Homer was still very much alive.
When Homer faked his death to get out of work, Patty and Selma showed up at the Simpson house holding a tombstone inscribed with "Homer J. Simpson. We are richer for having lost him." This gift as well as the fact they been saving for this since Marge's wedding causes her to angrily throw them out.
Her bad traits have not gone unnoticed by the populace of Springfield and even their family, her mother even stated "[they're] a sort of evil", having raised them her whole life.
Despite being identical twins, Patty and Selma have very different track records when it comes to finding dates. According to Marge, Patty chose a life of celibacy, while Selma had celibacy thrust upon her. Her standards are extremely low, as evidenced by her comments on Mr. Burns: "Single, eh? Well, he passes the Selma Test."
Selma actively sought out a husband, having been married a total of six times. Her current name evolved into Selma Bouvier-Terwilliger-Hutz-McClure-Discothèque-Simpson-D'Amico, after failed marriages to Sideshow Bob, Lionel Hutz, Troy McClure, Disco Stu, Abraham Simpson, and Fit Tony. Selma later began dating and later married Grampa Simpson, who previously dated her mother. Eventually, however, they realized they just cannot make it work because of his age and her career and called it quits. Selma once dated Hans Moleman but when the date finished, Selma imagined what life with Hans would be like and swiftly kicked him out of her car. Selma once made out on a park bench with Moe Szyslak after mistaking him for John Ritter; however they continued making out anyway, she later dated Moe a second time. She once dated Barney Gumble and may have hooked up with him again during "Grown up Halloween". Selma once claimed she and Kent Brockman made out on the date of 9/11.
Selma had a seemingly on/off relationship with Apu. When Apu was threatened with deportation Selma refused to marry him due to his long name. However she later slept with him after Homer and Marge's second wedding. Later still she was one of the women who bought Apu at a bachelor auction.
Unable to find companionship in a husband, Selma also sought to have children. At one point she was considering using a sperm donor. After babysitting Bart and Lisa for a day, she realized although she wanted someone to love, she was not ready to have children and wound up adopting Jub-Jub, her Aunt Gladys' pet iguana. Later however, a very menopausal Selma adopted a Chinese baby, Ling Bouvier. During the adoption process, Selma pretended to be married to Homer, since the Chinese government only allows children to be adopted by married couples. When the fraud was exposed, Selma managed to keep the baby, as a Chinese dignitary who had also been raised by a single parent became sympathetic.
As children, Selma and her sister Patty Bouvier were apparently domineering towards Marge and ridiculed her ambition of becoming an astronaut. Their relationship seems to have improved over the years. As adults, the Bouvier twins have a friendly relationship with their sister and frequently visit the Simpsons. They seem relatively fond of their nieces and nephew, but seem to like them more when they are young: "The older they get, the cuter they ain't." They, on occasion, watched the kids when their parents were out of town. However, Bart and Lisa would rather not be around when their aunts visit. Their idea of bonding with Lisa includes tutoring her in the belief men are pigs, using Homer as the prime example.
Their relationship with Homer remains one of mutual dislike (although Selma gradually grows to become more tolerant towards Homer and often experiences brief moments of kindness from both Homer and Selma) which involves a continuous exchange of insults from day one. They unfavorably compare their brother-in-law to a caveman, suggesting him capable of trading Maggie for beer and a nudie magazine.
Selma and Patty occasionally expressed their common hope Marge will eventually divorce Homer. They are often quick in pointing out his attempts at infidelity, and unsuccessfully offered to help Marge find a replacement for Homer (although Selma eventually gave in and helped Marge find Homer after seeing how truly unhappy she was without him). The twins went so far as to have a billboard built trying to convince voters to have Homer kicked out of Springfield.
Their dislike towards Homer even led to push Homer off a bridge, although they halfheartedly claim they did this in order to get him to his surprise party, conveniently on a boat passing under the bridge at the time.
Although Homer dislikes Selma and Patty, he showed occasional bouts of kindness towards them, and Selma in particular (much to the Twin's surprise), although, he often does this more because of his love for Marge than for any love for his in-laws.
Selma once took the Simpson children on a disastrous trip to the Duff Gardens theme park. Unable to control the children, Bart manhandles several animatronic characters and Lisa goes on a psychedelic trip after drinking "water" from an It's a Small World - like boat ride (later swimming naked in the Fermentarium and exclaiming "I am the lizard queen!"). After that, however, Selma returned home with the children and asked Homer how he does it, referring to raising children. She mentioned children and they exchanged a hug, implying Selma is slightly more capable of overcoming her dislike of Homer than Patty.