Penny is a fictional character on the American CBS sitcom The Big Bang Theory, portrayed by actress Kaley Cuoco. She is the primary female character in the series, befriending her across-the-hall neighbours Leonard Hofstadter( Johnny Galecki) and Sheldon Cooper( Jim Parsons ), two physicists who work at the nearby California Institute of Technology( Caltech ). Penny’s lack of advanced education, but outgoing personality and common sense drastically contrast with the personalities of the primary male characters in the series, even though she is considered part of their group. She is the love interest of Leonard, with whom she preserves a brief romantic relationship during the third season, which is later resumed in the fifth season and culminates in an participation at the end of the seventh season and a bridal at the start of season 9. Penny is the only main character of the prove whose last name has not been uncovered, although she has been occasionally referred to or resolved with the last name Hofstadter since her wedding.