Leonard Hofstadter started out as the relatable heart of The Big Bang Theory, but by the series finale, he had become one of the show's most divisive characters. While Sheldon, Howard, and Raj all experienced meaningful growth, Leonard seemed to become more jealous, manipulative, and unlikeable with each season. So what happened? From his relationship with Penny to the writers struggle to keep the show's original protagonist interesting, Leonard's character changed so dramatically he may be one of the biggest missed opportunities in sitcom history.