Penny

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

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 does 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 tends 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.

Relationships
Throughout the show Penny has been in a long standing relationship with Leonard, however she has been seen in the most other relationships out of all the main characters on the show. Still, outside from Kurt, Leonard has been the longest boyfriend Penny's ever had (at least throughout the series). It has become increasingly clear throughout season 4 that she still has feelings for Leonard and in the season finale, The Roomate Transmogrification, she, intoxicated, admits that she greatly regrets breaking up with him. Also intoxicated in the season 4 finale, she hooked up with Raj.

Trivia

 * Penny is 22 years old at the beginning of the show, which probably means that she is the youngest of the Gang.
 * Penny is a Sagittarius (meaning she was born between November 22 and December 21) but most people think that she's a water sign.
 * 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.
 * She is the only member of the main set of characters whose last name has not been spoken or revealed.
 * 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 (possibly 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.
 * It was mentioned in The Cohabitation Formulation that Penny once 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 is the only main character with her unknown last name.
 * She really hates Priya.