Leonard Hofstadter

Leonard moved in with Sheldon because the rent was cheap, and signed a roommate agreement full of clauses. The routine of comics, video games and takeout with Howard and Raj changes when Penny moves in across the hall. Leonard falls for her on day one and spends the whole season trying to get her attention.

Along the way, he goes out a few times with coworker Leslie Winkle, who treats him more like an experiment than a boyfriend. In the last episode, after Penny breaks up with her boyfriend, he finally works up the nerve to ask her out.

The first date doesn't click, and Penny says she'd rather stay friends. Leonard tries to move on: he goes out with Leslie again and dates Stephanie, a doctor who moves into his place without him noticing.

His mother, Beverly, a psychiatrist and neuroscientist, visits and analyzes her son like a patient. She gets along better with Sheldon than with Leonard. At season's end, Leonard leaves with his friends for three months at the North Pole, and his goodbye with Penny leaves something hanging.

Back from the Arctic, Leonard and Penny start dating. It's good but lopsided: Leonard is all in, and Penny is scared. When he says 'I love you' and she doesn't say it back, they break up and try to stay friends.

Leonard moves on and starts dating Raj's sister, Priya, a lawyer who doesn't like having Penny around. The long-distance relationship complicates everything, and at season's end he learns Priya is moving back to India for good.

A flashback from this stretch shows how life in 4A began: Sheldon saved Leonard's life during a rocket fuel experiment, and the explosion is why the building's elevator is always broken.

After Priya, Leonard and Penny slowly reconnect and get back together. He proposes at the worst possible moment and hears no, but the relationship survives. Leonard spends four months on a ship in the North Sea and comes home early to surprise her.

Penny quits her job to chase acting and shoots a cheap horror movie. Leonard visits the set, they fight and make up. Soon after, it's Penny who proposes, and he asks properly. The engagement is announced at season's end.

On the way to Las Vegas to get married, Leonard admits he kissed another woman on the North Sea ship. They marry anyway, livestreaming it to their friends, but the confession shakes Penny's trust and married life starts on careful footing.

The newlyweds still split their lives between the two apartments, and Sheldon struggles with the change. To please their families, the couple holds a second ceremony. Leonard's divorced parents, Beverly and anthropologist Alfred, clash, and Alfred also hits it off with Sheldon's mother, Mary.

Leonard, Howard and Sheldon build a gyroscope that becomes an Air Force project, and Leonard fears it will end up a weapon. In the end, the military takes it all. Soon after, he goes to Texas with Sheldon to patch things up with his brother and stands by his friend at his wedding to Amy. He spends the final seasons pushing for more respect at work and dealing with life in Sheldon's shadow.

With Penny, he wrestles with the question of kids: he wants them, she doesn't. In Stockholm, at Sheldon and Amy's Nobel ceremony, Penny reveals she's pregnant. Leonard watches his best friend thank the gang as his family and ends the story in apartment 4A with everyone.

Leonard is named after producer Sheldon Leonard and Nobel physics laureate Robert Hofstadter. Johnny Galecki was first asked to play Sheldon but felt he was better suited to Leonard.