Leonard Leakey Hofstadter, Ph.D., is an experimental physicist at the California Institute of Technology who rooms with Sheldon Cooper in Apartment 4A. Originally from New Jersey, Leonard attended Princeton University and was twenty-four years old when he received his doctorate with a Dissertation of the Year award for experimental particle physics. Having lived with Sheldon for ten years as of Season 6, he and Leonard are best friends; however, Leonard can become very agitated by many of his eccentricities and often feels obligated to explain Sheldon's behavior and serve as a mediator between him and other people in situations – he has been described as the straight man of the series. While Leonard, along with his geek friends and fellow scientists, is somewhat socially inept, Leonard would like to be better accepted outside of his social group. Leonard is better well-adjusted than his companions, being more willing to try different things. He appears to be best suited as a leader of the group, with Sheldon commenting how Leonard is the "Kirk" of the group. The writers have toyed with a romance between him and neighbor Penny, with their on-and-off relationship being a major force for drama.
Leonard was born on May 1980 in New Jersey. (Same month that Mt. St. Helen's erupted and "The Empire Strikes Back" came out.) Not much is known about his early life. He has two siblings: a younger brother named Mike and an elder sister that we don't know the name of ("The Maternal Capacitance", S2E15). At age eight his mother embarrassed him in front of the school by making him give back his science fair ribbon at the ceremony. He said it was his first science fair and that he had to give the ribbon back because he had done "Do Lima Beans React Better to Classical Music?". His brother had previously done "Do Lima Beans Grow Worse to Rock N' Roll" ("The Pants Alternative", S3E18) . His father was an anthropologist who spent more time with a 2-year-old Etruscan boy than with Leonard, who then said "I hate that boy" when he recalled this fact in The Fish Guts Displacement, S6E10). He did not celebrate his birthday or Christmas as a child (The Peanut Reaction, S1E16) due to his parent's wish to study the holidays for their "anthropological and sociological implications on society." (The Maternal Congruence, S3E11). The family wrote research papers for Christmas and they were returned graded with his stocking. ("The Santa Simulation", S6E11) As Leonard was a nerd, he was bullied badly as a youth and he stopped growing in the eighth grade. One Easter his mother sent him out on an Easter egg hunt without putting out any eggs to see how long he would search for them. Before he went to Princeton, his parents put him in advanced placement classes, and sent him to learn cello, "getting me beaten up enough", as he described himself ("The Peanut Reaction", S1E16). Johnny does play the cello.
Leonard is 5' 5" (1.65 m) with brunette hair and brown eyes. Penny thinks that his eyebrows are weird. His wardrobe includes black thick-framed glasses, a large variety of colored and patterned sweat shirts with hoods worn under a neutral colored jacket , brown trousers, and low-cut black Converse All Stars sneakers. Like Sheldon, Leonard often wears physics-themed t-shirts (under his other layers), but he has a much wider selection; many of his t-shirt themes deal with social issues such as renewable energy (wind farms) and recycling, as well as animal issues (rescue programs and themes around endangered species).
- (after he sarcastically tells Sheldon that they'll break into Penny's apartment again the next night to shampoo the carpet; Sheldon misunderstood it seriously) "For God's sake, Sheldon, do I have to... hold up a sarcasm sign every time I open my mouth?"
- "Okay, so we now have a socially-awkward genius in a room full of attractive, age-appropriate women."
- (whilst trying to get Kurt to pay Penny the debt he owe her) "I don't need backup. I have right on my side. And I'm wearing cargo shorts under my pants."
- Despite being the main character, Sheldon has more popularity than him. However, in his world, people like him more than they like Sheldon.
- Character has appeared in every episode including both pilots.
- Leonard wants to achieve a Nobel Prize someday.
- Leonard gets carsick unless he sits in the front.
- Leonard has trouble making eye contact, only being able to do so with Penny and the rest of his friends, which could be indicative of Asperger's Syndrome, ADD, or simply social anxiety.
- Leonard is the group's (not including Penny) Wii bowling 'Team Captain'.
- Leonard's last name (Hofstadter) is in honor of Nobel Prize Winning Physicist Robert Hofstadter, who worked at Stanford University in the field of Nuclear Physics.
- Leonard dislikes his name since it has "Nerd" in it, saying he prefers the name Angelo as it has "angel" and "Jello" in it, since he was high at the time.
- To prevent his pants getting stolen, Leonard wears cargo shorts underneath his pants whilst visiting Kurt's apartment for Penny's errands.
- Leonard is the only character in the show who has had five girlfriends (Joyce Kim, Leslie Winkle, Penny, Stephanie Barnett and Priya Koothrappali) until four of them broke up with him, leaving him with Penny. This situation is followed by Howard with four girlfriends (Christy Vanderbel, Leslie Winkle, Lisa and Bernadette Rostenkowski).
- Leonard along with his roommate Sheldon is named after the producer and director Sheldon Leonard.