Missy Cooper

Missy Cooper is Sheldon's fraternal twin sister, who is the direct opposite of her brother in many ways (besides being a female, she is not above physical confrontation, is very charming and amicable, and possesses a very cheeky sense of humor).

When Leonard, Howard, and Raj find out Sheldon has a sister and that she is quite attractive, they fall over each other trying to get her attention. Raj is inspired to begin taking an experimental medication to overcome his pathological shyness. Missy finds more of an ally with Penny, who has become slightly immune to the quartet's geekiness, and helps Missy extricate herself from them and let Missy hang-out in her apartment. ("The Porkchop Indeterminacy") Leonard, Howard, and Raj all knock on Penny's door and ask Missy out.

Trivia

 * She calls Sheldon by his nickname, "Shelly", as does their mother.
 * When they were kids, Sheldon turned her Easy-Bake Oven into a furnace for baking ceramic semiconductor substrates for homemade integrated circuits to make a robot to defend his room from her.
 * Missy tells people Sheldon is a "rocket scientist", which insults him because he is a theoretical physicist, which he views as much more prestigious and tells her that she might as well tell people that he is a toll collector at the Golden Gate Bridge.
 * Despite their differences, Missy is very proud of Sheldon, constantly bragging about her "brother the rocket scientist" to her friends.
 * When Leonard offered Missy to stay with them, she reveals that Sheldon does not like company, and he used to send his imaginary friends home at the end of the day when he was a child; he denies this, saying that they were "imaginary colleagues".
 * When aggressively approached for a date by Sheldon's friends, Missy rejects Leonard and Howard. However, when Raj makes his attempt she recognizes that his medication has worn off, so she changes her tactic and says, "Hi, cutie pie, I was hoping you'd show up." His mutism predictably kicks in, driving him away.
 * She once had a dog who made strange noises which she later put down.
 * It is revealed in the Pilot that she is a hostess at Fuddruckers when Sheldon is expressing concern of "committing genetic fraud" about donating sperm at a high-IQ sperm bank ("I have a sister who has the same basic DNA mix who hostesses at Fuddruckers.")
 * Her full first name may be Melissa, as Missy is a common nickname.
 * Her mother dismisses her as "dumb as soup" in "The Luminous Fish Effect"; though she is likely nowhere near as intelligent as her brother, she seems witty, presentable, and funny.
 * When she first meets the gang at Cal Tech, she tells them that she spent nine months with her legs wrapped around Sheldon's head.
 * She used to obligate Sheldon to do whatever she wants by pressing his testicles. She still does it when she gets mad at him.
 * Sheldon considers her, with his help in choosing a proper and approved-by-him mate, the future mother of a "Sheldon 2.0". She, however, doesn't see it that way.

Families

 * Father: George Cooper Sr.
 * Mother: Mary Cooper
 * Brother: Sheldon Cooper
 * Brother: George Cooper Jr.
 * Grandmother: Meemaw
 * Grandfather: Pop-pop
 * Uncle: Carl Cooper
 * Uncle: Uncle Edward