Penny

Penelope "Penny" (surname currently unknown) is Leonard and Sheldon's neighbor in 4B (across the hallway) in their apartment building.

Penny is the one of the female main characters in the show alongside Bernadette and Amy. During the first and third seasons, she was Leonard's love interest.

Physical Appearance
Penny is frequently referenced by the other characters as being incredibly attractive, a fact she seems to be aware of. Penny has blonde hair and green eyes. Wolowitz describes her as a "cheesecake-scented goddess". This description is fairly accurate, considering Penny is a waitress at the local Cheesecake Factory. Penny's height is 5'8", a fairly above average height, however she often appears short compared to the others, especially Sheldon.

Background
When she first met the guys in the Pilot, she was writing a movie about a girl from Lincoln, Nebraska, who travels to Los Angeles to become an actress but ends up working at The Cheesecake Factory instead. When Leonard asks if the movie is autobiographical, she replies no because she's from Omaha, Nebraska. The reason Penny is in the apartment building in the first place is because she split with her boyfriend of four years, Kurt, and was thrown out of their place. She says she still loved him but wanted custody of their television set. The shower in her apartment was broken at the time, much to Leonard and Sheldon's friends' delight (she used Leonard and Sheldon's shower and tended to wear nothing but a towel!). She's either ignorant of or all too aware of her effect on the boys, because within minutes of meeting them, she asked Leonard to retrieve her television set from her ex-boyfriend's apartment, which he immediately sets out to do. Penny is aware of Leonard's crush on her, and is hinted throughout the series that some of the feelings reciprocates. They later start dating and doing "stuff". She is messy and disorganized but appears to like it that way. Whenever Penny goes on a rebound relationship, she picks up an attractive man and then has 36 hours of meaningless sex over the course of a weekend; afterward, the relationship deteriorates. At the end of season one, Penny goes on a date with Leonard. They are both very nervous, but share a passionate kiss none-the-less.

In season two there is some initial awkwardness between Penny and Leonard. This is brought on by their insecurities concerning their relationship. Penny is worried that she isn't smart enough for Leonard and lied about graduating from community college because she felt insecure. The relationship quickly deteriorates and she goes back to dating different guys. She gives Leonard relationship advice when he's with Stephanie and they seem to develop a deeper friendship. She also starts to become geekier. She makes references to Star Trek, much to her own surprise, and plays Age of Conan. She also dates Stuart, who runs the comic book store, this annoys Leonard who thinks he and Stuart are pretty much the same person. When Penny and Stuart are making out she accidentally calls him Leonard and as a result, they stop seeing each other. This hints to her feelings for Leonard. When she learns he is going to the Arctic she is visibly upset, she buys him a blanket with sleeves and tells her door, after speaking with Leonard, she wishes he wasn't going.

In season three when the boys get back she has obviously been missing Leonard quite a bit and kisses him passionately the second she sees him. Their relationship has a rocky start because first they aren't left alone to be together, and second they seem to have some trouble getting used to being more than friends. They eventually start a stable relationship, which annoys Sheldon. Sheldon applies his knowledge of behavioral psychology and begins to train her using positive reinforcement techniques—i.e. chocolate—with a surprising amount of success. Leonard disapproves of this manipulative behavior and gets a dose of Sheldon's negative reinforcement techniques i.e. a squirt bottle.

Family
So far, the only member of Penny's family that has been seen is her father, Wyatt. Penny had a very close bond with him when she was a child, however, it is mentioned that it was a tomboy bond since he wanted another son instead of another daughter, so despite hating to get dirty, Penny tried to be good at sports. He used to call her 'slugger' until she bought her first training bra, and he just stopped calling her 'slugger' and didn't play catch with her anymore. Several other members have been mentioned too. Penny's sister is also mentioned, as having shot her husband, whilst both she and the husband were drunk. It is known that she has a nephew who turned 13 in the second season, and it's likely that he is the son of Penny's sister, as her sister is known to be married. When Christy comes into town, Penny reveals that she has a brother and at least one male cousin, as she claims that Christy slept with the former while being engaged to the latter. Her brother is also hinted to use (or at least make) meth, as Penny describes him as 'kind of a chemist' and told Leonard that he'll like him.

Leonard
Leonard became attracted to Penny the moment he met her and it didn't take her long to figure out that he had feelings for her. Penny shows no signs of interest in Leonard until The Hamburger Postulate when she hears of his failed attempt at a relationship with Leslie Winkle, though he sign is seen only by the audience. Despite these signs, she continues to date other men as Leonard continued to try and make her give him a chance. It isn't late in the first season that her and Leonard date, however they break up not long after when Penny fears he will get bored with her due to his high intelligence compared to her own. They remain friends although signs of their feelings for each other continue to show. After Leonard returns from his 3 month research trip at the north pole Penny immediately kisses him, having missed him greatly. They begin dating again and despite another rocky start, they are together for most of the 3rd season. They break up again after Leonard says he loves her and she doesn't say it back. They again stay friends throughout season 4 but it is strained as Penny struggles to control her feelings for Leonard as well as her jealousy towards Priya. While drunk, she admits to Raj that she shouldn't have ever broken up with Leonard.

Sheldon
Penny and Sheldon constantly irritate each other and get on each other's nerves. Sheldon is quick to take shots at Penny's intelligence, job, and seemingly failing career while Penny throws off his own personal routines. Despite their differences, Sheldon and Penny have become very close, and are quick to help each other. They have taken care of each other on multiple occasions, often times doing things they really don't want to do to make each other feel better. Major examples are when Penny takes care of Sheldon when he is sick and when she lets him stay at her apartment when he gets locked out, even letting him sleep in her own bed. In contrast, Sheldon was quick to come to Penny's aid when she called for help after falling in the shower and dislocating her shoulder, going as far as to drive her to the hospital despite not knowing how to drive. Many of these occurrences end with Penny singing the song "soft kitty" to Sheldon to make him feel better, although Sheldon sang it to her on one occasion as well. One of the biggest examples of their friendship is that despite his own rule to avoid physical contact with people. Penny is one of only 2 people Sheldon has hugged that aren't members of his family. The only other person being Wil Wheaton.

Howard
Upon meeting, Howard constantly hit on Penny in ways that made her deem him "disgusting." Their dynamic remains the same for the most part throughout the series but begins to change when Howard begins to date Bernadette. Although Penny is not a fan of him, her and Howard maintain a friendship, although they rarely hangout outside the group.

Rajesh
Like all other women, Raj was and is unable to speak to Penny unless he is drinking. They hangout within the group a lot and Penny occasionally attempts to tease him into talking to her. Raj begins to like Penny as the series goes on and the two end up getting drunk and appear to have slept together after though Raj later tells Penny that they did not have sex to her relief. Raj attempts to actually date her but she tells him she just wants to be friends. Despite their rough past, Penny attempts to find a girlfriend for Raj and comforts him when he may need it.

Bernadette
Penny and Bernadette work together at the Cheesecake Factory. She is the first person outside of the main group that Penny is seen to hang out with on a regular basis. They enjoy hanging out at Penny's apartment and going out together. They also hang out with the guys together once Howard begins bringing Bernadette around them.

Amy
When Amy begins hanging around with the group she attempts to befriend Penny. In no time she claims that her and Penny are best friends and calls her "bestie". When she initially hangs out with Penny and Bernadette her lack of social skills make it somewhat awkward, but as she spends more time with them they bond and the 3 become their own group within the group of guys. The two gradually form a tight friendship and continue to spend more time together and are quick to aid one another whenever the other needs it.

Trivia

 * Penny is 22 years old at the beginning of the show (in the early fall of 2007), which means that she is the youngest of the Gang.
 * Penny is a Sagittarius (meaning she was born between November 22 and December 21). In The Prestidigitation Approximation, Howard does a fake magic card trick and Penny turns over the fifth card, making the only possible birthdate in the span of the Sagittarius December 2nd (in the card trick, you take the sum of digits of the month and then add the day, so you get the number of the card you have to turn over). Considering these points, her birthday is December 2, 1984.
 * Penny grew up on a farm and rebuilt an entire tractor engine all by herself when she was 12.
 * She was Junior Rodeo champion.
 * Leonard and Sheldon's previous neighbor before Penny was a 200 pound transvestite named Louie/Louise with a skin condition according to Sheldon.
 * She is the only member of the main set of characters whose last name has not been spoken or revealed, but Bill Prady has said it eventually will be.
 * Penny claims to be a vegetarian but will eat fish and "the occasional steak. I love steak!"
 * Penny often smells like either vanilla (because she wears vanilla oil) or cheesecake (because she works at The Cheesecake Factory).
 * Penny has a sister, who once shot her own husband. They were both drunk.
 * Penny also has a brother, who slept with her friend Christy (who everyone refers to as 'the whore of Omaha'). She once says he's 'kind of a chemist' which probably means he's a drug dealer (most likely Meth), because it is mentioned at the same time as she mentions his court date.
 * She is very good at Halo the first time she plays it, which makes Sheldon extremely angry, because she beats him and he is a very sore loser.
 * Penny lives in apartment 4B.
 * Penny eats Chinese food with a fork and double dips her egg rolls (unappealing attributes, according to Sheldon).
 * Penny barely classifies her friend Christy as a friend because "friends don't get their friends' Care Bears all sweaty" (Christy and Howard had sex on them).
 * Her father used to call her "Slugger" until she got her first training bra, after that she wasn't "Slugger" anymore
 * Penny once got addicted to Age of Conan where she played as a Guardian named Queen Penelope and only overcame her addiction when she realised she'd accepted a virtual date from Sir Howard of Wolowitz (Howard).
 * She has a tattoo of the Chinese character for soup (汤) on her right buttock. She thinks it means "courage (勇)."
 * In The Staircase Implementation, it is revealed that Penny was almost a teenage mom.
 * She mentions in season 2 that she is 22 and she has a nephew who is 13.
 * Penny is very kind, but can become extremely fierce when provoked. She has yelled at Sheldon, Raj and Howard when they annoy her too much, and Leonard when she's angry.
 * Penny dated Leonard during most of Season 3. The relationship eventually came to an end during The Wheaton Recurrence after Leonard told Penny he loved her and she was unable to tell him she loved him too. They decided during The Spaghetti Catalyst that they able to stay as friends (for Sheldon's sake).
 * When under pain medication, she becomes almost as annoying as Sheldon (conjecture).
 * Penny believes in psychics, ghosts, astrology, voodoo, and other beliefs and traditions.
 * A reference to what may supposedly be her full first name was made in The Barbarian Sublimation, in which her online nickname to play Age of Conan was Queen Penelope.
 * She is from Omaha, Nebraska.
 * Penny is known to be very untidy. Sheldon once broke into her apartment to tidy it.
 * Penny went to community college, but dropped out for unspecified reasons.
 * The running gag of the show involves Sheldon knocking Thrice and saying "Penny". He repeats this thrice or until she answers the door. This is one of the most frequent scenes in the show.
 * She admits that she screwed up with Leonard by leaving him.
 * Whenever Priya says something bad about Penny, she says "That bitch!"
 * She really hates Priya.
 * According to Howard, he must obtain more than 1000 condoms for him to engage in coitus with Penny.
 * She had drunken sex with Leonard and Raj (Raj later revealed to Penny that they did not actually sleep together but keep it between them so Raj can save face) in last episodes in third and fourth season, furthermore both misunderstood that. It's unknown if this is coincidental or on purpose.
 * With the exception of his family and Wil Wheaton, Penny is the only person Sheldon has hugged.