Initially, Penny is the only female character in The Big Bang Theory. The boys' awkward interactions with her provide most of the series' humor, and her relationship with Leonard is the emotional center of the first seasons. She then forms a group of her own, affectionately named Penny's Posse by fans, and shares some unforgettable moments with Amy and Bernadette.
Despite being the de-facto female lead in the show, there are plenty of things about Penny's life that remain a secret. She keeps most of her background hidden, and her family only comes into the picture in later seasons. Fans keep discovering new things about her life, proving that not every stone in Big Bang has already been turned.
Her Birthday
Penny reveals in The Big Bang Theory's pilot episode that she's a Sagittarius. However, it's not until season nine that fans learn her actual birth date when Leonard reveals it's December 2nd.
The show did provide some hints about her birthday, the most prominent being in the season four episode "The Prestidigitation Approximation." Howard's card trick works by adding the month and day of a person's birthday and then choosing that card. Penny's card was number five, meaning her birthday could've been November 30, December 2nd, December 11, or December 20.
She Has A Sister Named Lisa
Unlike the other characters, Penny keeps most details about her family to herself. She mentions several times that she has a brother who's in prison and that he's "kind of a chemist." She also talks about having a sister, Lisa, but fans never get to see her.
In fact, Penny isn't very close with her family. She doesn't even let them know that she married Leonard and only does it after Amy and Bernadette force her into it. Furthermore, her dad, Wyatt, is also prone to keeping secrets from her, showing they are not the most communicative of families.
She May Have Been A Felon
Unlike her less fortunate friends, Penny was wildly popular in high school. She was Prom Queen and one of her school's most beloved students, or so she thought. In one episode, she realizes she was bully in high school. One of Penny's most reprehensible acts, which involved tying a girl to a post and leaving her in a cornfield, made Bernadette call her a felon.
Penny also reveals in another episode that she once stole a horse while drunk and hints that she also committed other questionable acts while under the influence. Penny isn't the only character to do something illegal on the show, but at least her young age could explain her recklessness.
She Has Two Tattoos At Least
In the unforgettable season 3 episode "The Adhesive Duck Duck Deficiency," fans learn that Penny has a Chinese character tattooed on her right buttock. She believes it says "courage," but Sheldon reveals it actually means "soup." He may have been wrong, because a previous episode establishes that he's surprisingly dense when trying to learn Mandarin and often speaks nonsense.
While broken up with Leonard, she also had another tattoo of the Cookie Monster, although she doesn't reveal where. Fans don't know if she has any more tattoos, but it seems likely.
She Could've Been A Promising Engineer
In the series' second episode, "The Big Bran Hypothesis," Penny reveals that she rebuilt a tractor engine by herself when she was twelve.
In several episodes, she expresses admiration for Howard's profession, once claiming that he "builds cool stuff that goes into outer space." She also shows a superficial interest in cars, although infamously neglects to care for her vehicle. Had she attended college, perhaps she could've been an engineer, but that is one big "what if."
She Could've Been A Successful Actress
Penny's acting career is a recurring joke throughout The Big Bang Theory. Her choices are often questionable, and her paying gigs are scarce. She's also a notoriously lousy singer, and several characters comment on her inability to act.
However, there is one project where Penny excelled as an actress. She was in an acting school production of A Streetcar Named Desire, where even Sheldon acknowledged her talent, calling her "remarkable." Even the infamous dud Serial Ape-ist earned her some fans, showing she did exhibit some charm in front of the camera. Perhaps all she needed was an opportunity to demonstrate just how talented she was.
She Has More In Common With Raj Than Anyone Else
Raj and Penny share some surprisingly wholesome moments. Because he's the most sensitive of the guys, he often enjoys spending time with the girls. When Raj moves in with Penny and Leonard, he ends up bonding with her through their shared love for yoga, shopping, and rom-coms.
They both love many of the same things, most notably shows like Sex and the City and books like Eat Pray Love. It's a shame that the writers never fully explore their relationship because it could've been one of the show's most intriguing and compelling.
She's More Intelligent Than Anyone Realizes
Sheldon once says that Penny "isn't as academically inclined" as the rest of the gang, but she does have a cleverness that even he envies. Penny is very intuitive and savvy, and is the most emotionally intelligent of the group.
Penny also displays her strategic mind numerous times. She excels at many strategy-based games like Age of Conan and Halo and beat Leonard at chess on her first try. He even states that she "plays chess like Bobby Fisher." Penny lacks the same level of educational pedigree that her friends have, but she does have a remarkably shrewd intellect that she seldom got to exploit.
She Is (Arguably) Sheldon's Best Friend
Penny and Sheldon's relationship is the best. The two have a completely platonic bond that grows over the seasons until they become like brother and sister (and occasionally, mother and son). The two not only tolerate each other but actually learn to enjoy their company, discovering they have more things in common than they ever imagined.
In many ways, Penny is Sheldon's best friend. She understands him better than anyone and knows the best approach to deal with him. The scene they share in the bathroom during his birthday party shows that they accept and love each other not in spite, but because of all the things that drive others crazy.
Her Last Name
For years, and wondered what Penny's last name was. The show never even hints at the answer to the mysterious question, and Sheldon refers to her as "Penny Hofstadter" during his Nobel speech during the series' final episode.
However, eagle-eyed fan and Redditor Quell_Alma found Penny's last name in a package sticker during the Penny Blossoms episode. It turns out Penny's last name is Teller, a possible reference to magician duo Penn & Teller, and may have only been a placeholder until the showrunners decided on a more definitive surname. Alas, that never happened, so fans will forever call her Penny Teller Hofstadter.