These characters appear in several episodes. The list is sorted by chronological order of appearance, considering their first appearance in the show.
Kurt (Brian Patrick Wade): A tall, muscular intimidating bully, Kurt is Penny's ex-boyfriend at the beginning of the series. In the pilot episode Leonard and Sheldon went to his apartment and tried to retrieve Penny's TV set from him, but they did not succeed and returned home without pants. Penny left him because he cheated on her, but he was still invited to her Halloween party, which he dresses up as a caveman. When Penny experienced financial difficulties, Leonard and the guys tried to convince Kurt to pay Penny back money he owed her; at first Kurt refused, but eventually he paid Penny back to date her again.
Dr. Eric Gablehauser (Mark Harelik): The head of the Physics Department, Dr. Gablehauser is the group's boss at the university. In his first appearance, he fired Sheldon after Sheldon insulted his intelligence. Eventually, Sheldon was rehired because Gablehauser became romantically interested in Mary Cooper, Sheldon's mother. Gablehauser refers to Sheldon, Leonard and Rajesh as doctors, but to Howard as "mister" because he lacks a Ph.D (but carries a Masters Degree). Dr. Gablehauser was also responsible for introducing child genius Dennis Kim to the university and for hosting the Physics Bowl.
Mary Cooper (Laurie Metcalf): A devout Christian from Texas, Mary is Sheldon's mother. She has two other children besides Sheldon; Sheldon's fraternal twin sister, Missy and his brother George, who is shown via a family tree in "The Cruciferous Vegetable Amplification" to be three years older than Sheldon and Missy. She appeared in the fourth episode of the first season, the season three premiere, the third episode of season four, and the sixth episode of season five. Mary herself is not intellectual but is very wise, and prone to making politically incorrect comments; she appears to be an extremely good mother and is the only one who has ever been able to control Sheldon. Leonard described Mary as Sheldon's "Kryptonite". When Penny and Sheldon engaged in a fierce dispute, Penny called for Mary's help, who subsequently called and scolded Sheldon about his actions. She often says politically incorrect statements, such as referring to Chinese characters as "kung fu letters" when Leonard takes her to a sushi restaurant, or saying that "our Indian has the drinking problem" when she sees Raj drunk.
Mrs. Wolowitz (voiced by Carol Ann Susi): Howard's overbearing Jewish mother, she is never seen on-screen but her voice is heard when he is at their house or when he talks to her on the phone. She talks to Howard always by yelling at him from another room, which results in awkward conversations with Howard yelling back at her. Mrs. Wolowitz seems to be oblivious about Howard's work as an engineer, and talks to him like he is still a child. She frequently refers to Leonard, Sheldon and Raj as Howard's "little friends" when yelling to him (as in, "I made some cookies for you and your little friends!") She makes Howard's life miserable at home, which prompts him to call her a "crazy old lady". However, in one episode, Howard attempted to move out, but had serious delusions that his mother was trying to force him to stay, when in actuality, she was urging him to move out, so he stayed. In the season 4 finale, she wants to "play doctor" with Raj when he asks to sleep over. When Howard revealed that he proposed to Bernadette and she accepted, she collapsed, causing Howard to panic (and break his arm trying to barge into the bathroom to help her), and the timing leads him to believe that she may not have liked Bernadette and had suffered a heart attack from the shock of the news. In actuality, her fainting was, coincidentally, due to food poisoning from the food she ate while meeting Bernadette, and was completely unrelated to Howard's news.
Dr. V.M. Koothrappali (Brian George) and Mrs. Koothrappali (Alice Amter): Raj's parents in India, they communicate with their son via video chat and constantly try to arrange dates for him. They want their son to marry a woman of Indian descent, and give them grandchildren. Mrs. Koothrappali is especially worried that, despite Raj being old enough to marry, the closest they have to a daughter-in-law is the "Jewish boy", Howard. Although in many episodes Raj mentions that he grew up in poverty, his friends remind him of the contrary by pointing out that Dr. Koothrappali is a gynaecologist and drives a Bentley. To this, Raj replies "It was a lease". The Koothrappalis enjoy Doogie Howser reruns, which are apparently new to India. Other than Raj, they also maintain contact with Priya Koothrappali via video chat whenever she is out of India. They also had some anger towards Leonard when it became apparent that he was dating Priya without their knowledge.
Dr. Stephanie Barnett (Sara Rue): A doctor and highly distinguished surgical resident at Fremont Memorial, Stephanie is first seen as Howard's date, but later becomes Leonard's girlfriend. When Howard's plan to let her drive the Mars rover failed, she left with Leonard and the two immediately became attracted to each other.Sheldon began meddling in Stephanie and Leonard's new relationship in an effort to consolidate it, since in his eyes, Stephanie was the only "tolerable" mate Leonard had had. After several weeks of dating, Stephanie officially began living with Leonard in his and Sheldon's apartment. However, after much hesitance, Leonard told her to move out because he was uncomfortable with the pace of the relationship. Although they were not seen breaking up on camera, evidence suggests their relationship ended. Series co-creator Bill Prady confirmed this in an interview, stating: "Stephanie was a chance for Leonard to learn that just because someone loves you, doesn't mean you'll love them back". Stephanie did her medical internship at Lawrence Memorial in Galveston, Texas, which is where Sheldon was born.
Dr. Beverly Hofstadter (Christine Baranski): Leonard's overly analytical mother, Beverly is a neuroscientist as well as a psychiatrist. Between her neurotically strict speech patterns, lack of social conventions, and attention to detail, she is Sheldon's female equivalent. The two of them share an odd non-romantic attraction to each other, which culminates in her kissing him after becoming drunk. Though still married to Leonard's father, she revealed in an appearance that the two "are getting a divorce". She had frequently implied that the two have not had "coitus" since Leonard was conceived. She diagnosed Raj with selective mutism and considered Raj and Howard's relationship to be an "ersatz homosexual marriage". Beverly mentioned that Leonard's brother and sister are more successful in their respective fields than he is. Ironically, as displayed in "The Maternal Congruence" episode, Sheldon is closer to Beverly than her own son is. In both 2009 and 2010, Christine Baranski was nominated for an Emmy Award for Outstanding Guest Actress in a Comedy Series for this role. She returns via Skype for the season 5 premiere.
Barry Kripke (John Ross Bowie): An unlikable co-worker of Leonard and Sheldon's, who works in plasma physics, Kripke has a case of rhotacism where he pronounces the letters "R" and "L" as "W" in much the same way as Elmer Fudd from Looney Tunes. In his first appearance, he pitted his robot, the Kripke Krippler (or, as he called it, the "Kwipke Kwippwuh"), against the guys' robot, M.O.N.T.E., in an unofficial robot fight. On another episode, Sheldon attempted to befriend Kripke in order to gain access to an open science grid computer to carry out research, but it turned out to be futile, as Kripke had no control of the computer as Sheldon had thought. Kripke continued his antagonism towards Sheldon, when he pulled a prank on Sheldon when the latter was a guest on NPR's Science Friday. Sheldon tried to befriend Kripke another time when he was feeling left out from his friends, but Barry connected more with the others in Sheldon's new proposed group. Kripke purchases an iPhone with voice recognition technology, but due to his rhatacism, the device is incapable of understanding his verbal "wequests". In Season 5's "The Rothman Disintegration", Kripke and Sheldon battle over a retiring professor's office.
Stuart Bloom (Kevin Sussman): Stuart runs the comic book store that the guys frequently go to. He is also a nerd, but he has a talent for drawing, is a graduate of Rhode Island School of Design and possesses a few more social skills than they do. During Stuart's first appearance, the guys brought Penny along to the store and he managed to ask her on a date. The date went fine until Stuart was dragged into discussing comic books by Sheldon, leaving Penny bored. On a second date with Penny, Stuart asked for Leonard's advice, but feeling threatened, Leonard ultimately gave him bad advice. The date went fine, but when Penny was getting romantic with Stuart she mistakenly called him "Leonard", leaving him devastated. Stuart was the partner of Wil Wheaton in the card game tournament, where they defeated the pair formed by Raj and Sheldon. As Stuart runs a comic book store, he has an almost encyclopedic knowledge of comic books and superheroes. In the Season 4 episode "The Toast Derivation", he implied he was in financial trouble and that the comic book store is now also his home. In "The Justice League Recombination", he stated that he works 70 hours and clears an average of $1.65 per hour after expenses, so his income per week would be $115.
Wil Wheaton (portraying an antagonistic version of himself): In the season 3 episode "The Creepy Candy Coating Corollary", Sheldon enters a card game tournament (Mystic Warlords of Ka'ah) to confront Wil Wheaton over an incident that occurred in 1995 when Sheldon was devastated because Wheaton (Wesley Crusher in Star Trek: The Next Generation) failed to show up at a fan convention that Sheldon took a 10-hour bus ride to attend (in which he had to "twice violate his person rule of no relief on a moving vehicle"). In the final match, Sheldon is about to defeat his hated rival, but Wheaton tells Sheldon that the reason he missed the convention was because his grandma had died. After Sheldon wastes his last opportunity to win under sympathy, Wheaton reveals that he lied about his grandma allowing him to win the game. Wil Wheaton makes a second appearance in the episode "The Wheaton Recurrence", where he uses gamesmanship again as seen when he breaks up Leonard and Penny's relationship in order to win a bowling competition against the main characters. In "The 21-Second Excitation", Wheaton appears at a screening of Raiders of the Lost Ark, in which he uses his celebrity status to go to the front of a long line. This antagonizes Sheldon, who derides him as the "Jar Jar Binks of the Star Trek universe". Wheaton and his entourage effectively prevent Sheldon and the guys from attending the screening when the last seat in the theater goes to the person directly in front of them in line. Ultimately, Sheldon steals the prints of the film and Wheaton leads a mob of angry Raiders fans after Sheldon, Howard, Raj and Leonard. In the Season 5 episode "The Russian Rocket Reaction", Wheaton appears to be genuinely sorry for his past actions, and gives Sheldon a signed Wesley Crusher action figure as a way of thanking Sheldon for being a fan. His gesture is met with a renunciation of his "mortal enemy" status, and a hug from Sheldon. Along with LeVar Burton, Brent Spiner and George Takei, Wheaton makes Star Trek the most represented franchise on the show in terms of guest appearances.
Zack Johnson (Brian Thomas Smith): A dim-witted beefcake-type who Penny dates after her second break-up with Leonard. Leonard invites Penny and him up on the roof to watch his experiment of bouncing laser beams off the moon; Zack has no clue what is happening. Later, he and Penny go out on a date, but she is so put off at how stupid he is that she runs back to Leonard to have sex, saying that Leonard has ruined her ability to date stupid men. In a later episode, he runs into Penny, Bernadette, and Amy at a restaurant, and Amy becomes physically attracted to him. Unlike Penny's other boyfriends, he considers Leonard, Howard, Sheldon, and Raj (whom he calls "the science guys") to be "cool" and makes a genuine effort to get to know them, considering them his friends. After accompanying them on a trip to the comic book store (he and Raj bond over a mutual fondness for Archie comics), he enters a New Year's Eve costume party with them, portraying Superman in their version of the Justice League. He works as the menu designer for restaurants either owned or who employ his father's company to design their menus. More recently, he seems to have befriended Barry Kripke and Stuart as well.
Priya Koothrappali (Aarti Mann): Raj's sister, a lawyer who graduated at the top of her class at Cambridge. Introduced in the fourth season, it is implied upon her first appearance in "The Irish Pub Formulation" episode that she and Leonard had a relationship five years earlier (without her parents' or Raj's approval) when she was in town, and that Leonard had been much more invested in the relationship than she. Leonard had even offered to relocate to Delhi to be with her. He tries to keep their relationship a secret from the guys, but Sheldon discovers Priya in their apartment. Leonard eventually reveals to the rest of the guys that he had spent the night with Priya, as he is unable to bear the burden of keeping the secret. Priya returns to Los Angeles for business in "The Cohabitation Formulation", and she and Leonard decide to try and have a real relationship, over the protests of Raj. Priya soon becomes jealous of Penny, and asks Leonard to stop spending time with her. Priya contests Sheldon's roommate agreement with Leonard, employing her legal background to deny Sheldon power over Leonard, but Sheldon drafts a new agreement and blackmails Leonard into signing it. In the fourth season finale, it is revealed she is moving back to India and has not told Leonard, which Leonard interprets as the end of their relationship and causes him to expose their secret relationship to her parents. Although they are then separated with Priya moving back a month earlier than expected, they nonetheless remained in-touch via webcam, having a cyber-dinner date/breakfast date. At Howard's suggestion, Leonard and Priya also attempted to simulate sex via webcam, but when it came time for her to remove her clothing, Leonard's screen froze up because of exceeding the bandwidth. He later managed to get cue cards to help himself with a seduction speech to Priya, but while delivering it to her, he inadvertently revealed to her parents that they were cyber-dating (they were in the same room as her when he called) with them bitterly asking for him to continue with the speech. After Leonard tells Priya about his attraction to a comic book enthusiast named Alice, Priya admits to recently sleeping with her ex-boyfriend which visibly upsets Leonard. In the next episode, Leonard refers to himself as "single", suggesting he and Priya have broken up.