Leonard Hofstadter is an experimental physicist at Caltech in Pasadena. Leonard is currently engaged to Penny, whom he lives across the hall from with Sheldon Cooper.
Although Leonard has many geeky interests and hobbies, he has been the most willing of the guys to try and socialize with people. Leonard's interest in Penny is the reason she became a part of their social group and helped bring the guys out of their shell.
Leonard is known for his black-framed glasses, natural-colored jackets and hoodies, which he usually wears over superhero or physics t-shirts, and he typically wears low-cut black Converse All Star sneakers.
Leonard is from a high-achieving family and has often felt unloved and under appreciated. All the members of his family are accomplished scientists, except for his younger brother who is a tenured law professor at Harvard.
Leonard's mother, Dr. Beverly Hofstadter, is a psychiatrist and neuroscientist. At best her mothering style would be described as cold and distant. She did not heap love or praise on Leonard as a child. Growing up, Leonard's birthdays were not celebrated, and for Christmas he was made to prepare a scientific paper for Santa which his parents would grade. Because of the lack of affection from his mother, Leonard developed a hugging machine.
Leonard's mother has formed a bond with Sheldon, whose personality is similar to hers, much to Leonard's annoyance. Beverly now keeps in closer contact with Sheldon than she does her own son, though Leonard is unlikely to want to hear from his mother more often than he already does.
Relationship with Penny
Leonard's infatuation with Penny began the day he saw her move into the apartment across the hall. Despite Sheldon's disinclination to socialize with their new neighbor, Leonard invited her over for lunch. After a year of living across the hall from each other, Leonard finally asked Penny out - a proposal she accepted. After an "okay" date, Penny broke things off as she feared she wasn't smart enough to date a guy like Leonard. The pair went back to dating other people.
When Sheldon was offered the chance to lead a three month research expedition in the Arctic, he invited Leonard, Howard and Raj along to help him. When Penny found out Leonard would be away for months, her feelings for him became clear to her, although she couldn't build up the courage to tell Leonard not to go. When Leonard returned from the three-month trip, Penny kissed him and they soon started dating.
Leonard and Penny's relationship came to an end when Leonard told Penny he loved her and she couldn't say the same back. Despite the awkwardness, the pair vowed to remain friends even as they began to date other people. Leonard and Penny's friendship suffered when he dated Priya, who told him he shouldn't hang around with Penny any more.
After his relationship with Priya dissolved, Leonard spontaneously asked Penny out on a date, which she accepted. After a beta test where they tried to iron out the kinks in their new relationship, Leonard and Penny got back together and have remained a couple since. When Leonard was jealous about Penny having a male friend, Penny finally told him he's the one she loves - the first time she was able to say she loved him. Later, Leonard left for a four month expedition in the North Sea, with their relationship on a solid footing.
Following a setback in her acting career, Penny proposed to Leonard while drunk. Leonard hesitated to answer, and later feared their relationship was over, but Penny assured him he made the right decision. Following another career disappointment, Penny vowed to start making better live choices, deciding she and Leonard should get married. At first, Leonard thought Penny only wanted to get married because of her career problems, but she told him she now knew what was important in her life. After Leonard was convinced she really did want to get married, he got down on one knee and proposed to Penny, handing her an engagement ring he'd been carrying around.
Following their engagement, the couple have not yet set a date for the wedding. Leonard and Penny are considering living together, though they are both sympathetic to how that will affect Sheldon.
Not long after Penny moved in to apartment 4B, Leonard's hopes of getting together with her were dashed when he saw her with another guy. In response, Leonard asked out Leslie Winkle, a fellow scientist from the university who greatly irritated Sheldon. Leonard's dalliance with Leslie was short-lived as she wasn't interested in a long-term relationship. A year later, after Leonard's ultimately unsuccessful date with Penny, Leslie decided she wanted a long-term partner but decided Leonard wasn't a suitable mate because of a scientific disagreement.
Leonard had a relatively short relationship with Dr. Stephanie Barnett, a girl Howard initially picked up and had hoped to impress by letting her drive the Mars Rover. Howard was angered when Leonard stole his girl, though Stephanie told Leonard that Howard never had a chance with her - he was wearing an eye patch, after all. Leonard and Stephanie's relationship was going well - even Sheldon liked her - until Leonard realized that Stephanie had moved in without him noticing. Fearing their relationship was moving too fast, Leonard tried to break up with her - but each time just led to sex.
Later, when Raj's sister Priya moved to Pasadena, Leonard and her started a relationship after their previous one night stands. During their time together, Leonard's friendship with Raj suffered, as he was unhappy seeing his friend date his sister, while Priya told Leonard not to see Penny any more. Leonard was angered when he found out Priya was moving back to India, but they tried to keep a long distance relationship going.
During his long distance relationship with Priya, Leonard met a comic book artist, Alice, and hit things off. Leonard and Alice shared a kiss, but Leonard was torn over whether he should have sex with her while in a relationship with Priya. Leonard ultimately decided to be a "good guy" and not sleep with Alice. When Leonard told Priya about Alice and his decision not to sleep with her, Priya revealed she had already cheated on Leonard with another guy, ending their relationship.
Living with Sheldon
Although Sheldon is Leonard's best friend and roommate, living with him and being his friend is not without its troubles for Leonard. Leonard does his best to appreciate Sheldon's quirks and eccentricities, but Sheldon can be very annoying.
Leonard not only has to put up with Sheldon's quirks and demands, he has to perform a lot of errands for Sheldon, too. Sheldon has, at times, come through for his buddy, including the time he saved Leonard from exploding rocket fuel.
When Professor Proton asked Leonard why he put up with Sheldon, Leonard told him how Sheldon can be aggravating but he isn't doing it on purpose. Leonard said Sheldon is loyal, trustworthy and that they have fun together. Leonard called Sheldon the smartest person he knew, and admitted that Sheldon's a little broken and needs him, and that he needs Sheldon, too.
Leonard has, at times, threatened to leave Sheldon, rather than submit to his will or roommate agreement. When Sheldon annoyed him by waking him up in the middle of the night for an apartment safety drill, Leonard nullified their friendship. Later, after Sheldon spoiled the Harry Potter books for Leonard, he temporarily moved in with Penny.
Leonard has close friendships with Howard and Raj, both of whom were his friends before he met Sheldon. Leonard has at times been Howard's wingman, though after spending a night on the town with Howard, Leonard started to question whether Howard saw nearly as much action as he claimed. After Howard called on a pact he had with Leonard, he asked Penny to set Howard up with one of her friends. Penny set him up with Bernadette, at that time her co-worker at the Cheesecake Factory. Bernadette is now Howard's wife.
Leonard's friendship with Raj encountered trouble when Leonard started dating Priya. Raj was very unhappy about the relationship and this led to an evening where he got drunk with Penny, later ending up in bed with her. Leonard was angry at Raj for seemingly wrecking his chances with Penny, though things later cleared up.
Leonard is an experimental physicist at Caltech. In his career, he has occasionally worked with Sheldon, often with mixed results. After Sheldon and Leonard wrote a paper on super-solids, Sheldon received a letter asking them to present their paper at a physics conference. Sheldon threw the letter away without asking Leonard, who later decided to present the paper without Sheldon. Sheldon showed up at Leonard's presentation and derailed it, leading to a physical fight between him and Leonard that ended up on YouTube.
Leonard joined Howard and Raj as part of Sheldon's team to study magnetic monopoles in the Arctic. Later, after Howard recommended him to Professor Stephen Hawking, Leonard was part of a North Sea expedition to test hydrodynamic simulations of black holes.
After Leonard had an idea for a new way to look at super fluids, he and Sheldon co-wrote a paper and published it online. Later, when the paper was mentioned in the Scientific American, Leonard was upset that his contribution to the paper was overlooked.
Professor Proton: Can I ask you a question?
Leonard: Yeah, sure.
Professor Proton: Why do you put up with Sheldon?
Leonard: Oh, you know because we're friends.
Professor Proton: Why?
Leonard: Wow, you ask really hard questions. Look, I know he can be aggravating, but what you have to remember is he's not doing it on purpose, it's just how he is. But he's also loyal and trustworthy and we have fun together.
Professor Proton: You know you're describing a dog?
Leonard: He did bite me once. But in his defense, I came up behind him while he was eating.
Professor Proton: They hate that.
Leonard: Sheldon is the smartest person I have met. He's a little broken and he needs me. And I guess I need him too.
Professor Proton: Why is that?
Leonard: Boy, you will not let this go, will you?
Leonard: What would you be if you were attached to another object by an inclined plane, wrapped helically around an axis?
Leonard: There you go.
Sheldon: I've changed. Like the frog who's put in a pot of water that's heated so gradually, he doesn't realize he's boiling to death.
Penny: Or you're the frog who's been kissed by the princess and turned into a prince.
Leonard: Or you're just a tall, annoying frog.
Leonard: You know what, this bottle was meant to celebrate an achievement. Let's make a pact. When one of us gets their first big breakthrough, we'll celebrate by opening this bottle and toasting Professor Abbott.
Howard: I love that.
Raj: Me too.
Leonard: Then, of course, rubbing our success in Sheldon's face.
Howard: Well that's the best part.
Sheldon:He'll be back.
Leonard: (Through the door) Of course, I'll be back, I live here!