Leonard's Sister

Hayley Hofstadter is the oldest child and only daughter of Mr. Hofstadter and Beverly Hofstadter, as well as the sister of Leonard Hofstadter and Michael Hofstadter. She has never appeared on the show, but has been mentioned a couple of times. It is known that she is a cutting-edge medical researcher working with s to cure.

She is older than Leonard, as Leonard mentions in "The Cohabitation Formulation" that she is 38 years old and married, after Raj is mad at him for spending the night with his sister Priya and says in retaliation that he shall have his way with Leonard's sister.