Penny

Fresh off a breakup with her boyfriend Kurt, Penny moves in across the hall from Leonard and Sheldon and soon knocks to borrow their shower. The two physicists and their friends have no idea how to act around her: Howard hits on her nonstop and Raj can only talk after a drink, but Penny enjoys the company and starts eating with them.

She learns to handle Sheldon's quirks, sings him Soft Kitty when he's sick and notices Leonard has a crush on her. After one more letdown from her ex, she agrees to go out with Leonard at season's end.

The date with Leonard doesn't turn into a relationship: Penny worries she isn't smart enough for him and says she'd rather stay friends. She tells Sheldon her secret, and he agonizes over keeping it. She dates Stuart, the comic book store owner, and puts up with Leonard's girlfriends.

Bit by bit, she becomes a fixture in apartment 4A, between game nights, Chinese food and Sheldon's chaos. When the guys leave for three months at the North Pole, Penny says goodbye to Leonard with a long hug.

Back from the Arctic, Penny and Leonard start dating. She likes him, but panics when he says 'I love you' too soon and can't say it back. They break up. Penny goes back to Zack, a handsome, not-so-bright boyfriend, and dumps him too.

The group grows: Penny sets Howard up with Bernadette and gains a best friend in Amy, who idolizes her. At work, she moves behind the bar at the Cheesecake Factory. Watching Leonard date Priya bothers her more than she admits, and the season ends with Penny waking up next to Raj after a night of drinking.

The night with Raj turns out to be nothing but drinking, and Penny and Leonard get back together. She turns down a badly timed proposal, but the relationship matures. Determined to go all in on her career, she quits the Cheesecake Factory and lands a part in a cheap horror movie, Serial Ape-ist 2.

The movie opens no doors, and it's Penny who proposes to Leonard. Soon after, she gives up acting and, with Bernadette's help, becomes a pharmaceutical sales rep. She turns out to be good at it and, for the first time, has a steady career.

Penny and Leonard marry in Las Vegas, but his confession about kissing another woman on a ship still stings. They still have to work out how to split life between the two apartments, with Sheldon in the middle, and they hold a second ceremony for their families.

That's when Penny's family really shows up: her father, Wyatt, who raised her almost like a son, her mother, Susan, and her brother Randall, who has a rocky history with drugs. Penny tries to keep her family from embarrassing her in front of the Hofstadters, and the wedding becomes a chaotic reunion.

Penny makes it clear she doesn't want kids, and Leonard accepts it even though he does. The gang changes around her: Bernadette has her second child, Amy marries Sheldon, and the couple wins the Nobel.

On the trip to Stockholm for the ceremony, Penny reveals she's pregnant, a surprise she and Leonard had been keeping. The story ends with Penny, the neighbor who once knocked to borrow a shower, in apartment 4A with the family she chose.

In the original pilot, which CBS passed on, the neighbor was Katie, played by Amanda Walsh. In the reshot pilot, Kaley Cuoco landed the role of Penny. Her maiden name was never said; a package addressed to 'Penny Teller' appeared on screen, but the producers said it isn't canon.