Missy Cooper

Missy Cooper is Sheldon's fraternal twin sister, who is the direct opposite of her brother in many ways (she is a female and not above physical confrontation).

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 seems to be the only one who was partly successful in this endeavor. 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. ("The Porkchop Indeterminacy")

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 silicon 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".
 * She once had a dog who made strange noises.
 * 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."