Missy Cooper is Sheldon's fraternal twin sister, who is ironically the direct opposite of her brother in many ways, and though she tends to torment him. She is the sister-in-law of Amy.
Missy Cooper, when she was a child, was once a mischievous and bratty twin sister of Sheldon Cooper, where she does not share the same intelligence as her twin brother, but she does possess a cheeky sense of humor. She is disrespectful to her twin brother and tends to fight over her brother to try and get the attention of her parents. Despite having a dislike of her twin brother and her pleasure to tease him, she does care for Sheldon at times and he does the same for her. She has a keen interest in baseball and has a crush on a boy in baseball. She shows no interest in homework at all, only to watch cartoons on TV all the time.
When she became an adult, she became more mature and is now tolerant with her brother's upper class jobs as a physicist (Sheldon) and the tire entrepreneur (Georgie) as she now works as a hostess for a living. Despite this, she has also grown to be a attractive woman where this results in Leonard, Howard and Raj attempt to hit on her, but she turns them down nevertheless. Though, she still has to deal with Sheldon's difficulty and stubbornness where at one point, she got her twin brother in the crotch to emphasize her point.
Missy Cooper is a slender woman who is the same height as her twin brother. She has fair skin with long brown hair, blue eyes and pink lips.
When she was a child, she sometimes put her hair in pigtails and sometimes wears red dungaree dress with a differently colored buttoned shirt and brown cowgirl boots. Her choice of clothing depends on either wearing short-sleeved buttoned shirts with jeans or overalls.
When she grows in a woman, she wears a black and purple floral dress with black pumps. Though in later seasons, she appears wearing a shirt and pants.
In The Porkchop Indeterminacy, Missy travels to California to attend her friend's wedding, but she stops by a CalTech to get her brother to sign some of her papers that relate to her father's estate. However, the guys at CalTech become attracted to her and soon she was introduced to the guys by Sheldon. He welcomes his twin sister to the apartment building where she meets and befriends Penny. Because the guys are flirting with her, Missy joins Penny in her apartment before the guys start fighting over each other. Sheldon deems the guys unsuitable and is caught out by Missy when she and Penny witness his plan on finding his sister a suitable mate. Missy talks to Sheldon in his bedroom and reminds him of how they handled conflict when they were young. As a result, Missy kicks Sheldon in the groin, forcing him to agree that she can date whoever she likes. Leonard and Howard try to charm her, but they were turned down. Finally, before Raj can talk to Missy, his medication wears off and is unable to speak to her, leaving him defeated. Before leaving the next day, Missy tells her brother that she is proud of him and brags to her friends about her brother the "rocket scientist", offending her brother, wondering why she doesn't just tell them that he's a toll-taker at the Golden Gate bridge.