These characters appear in fewer episodes, most in just one. The list is sorted by chronological order of appearance, considering their first appearance in the show.
Althea (Vernee Watson) A character that made an appearance in the pilot episode as an attendant at a sperm bank, and then made three appearances as a emergency room nurse in The Peanut Reaction episode (season 1), The Robotic Manipulation episode (Season 4), and The Werewolf Transformation (Season 5). The name of the character is never used on the show, only on her nametag and in the credits.
Chen (James Hong): The owner of a Chinese restaurant the guys frequent.
Cheryl (Erin Allin O'Reilly): A girl at Penny's Halloween party who ends up sleeping with Raj.
Christy (Brooke D'Orsay): An acquaintance of Penny from Nebraska who had slept with enough of her relatives to make her "family", Christy suddenly decided to move in with Penny. Being promiscuous, Christy ends up sleeping with Howard, who in turn invites her to move in with him (and his mother). Mrs. Wolowitz and Christy end up having a fight, and the latter leaves.
Lalita Gupta (Sarayu Rao): A childhood acquaintance of Raj, she is now a dental student at USC. Raj's parents set up a date for their son with her. During the date, Raj can only speak to her after drinking an alcoholic beverage (a "grasshopper"). Sheldon insists she bears a remarkable resemblance to Princess Panchali, an Indian princess in a children's story. Lalita is flattered at Sheldon's comments and attention. Eventually, Lalita leaves the obnoxious and drunk Raj to have dinner with Sheldon. Sheldon does not see her again because he "already has a dentist".
Toby Loobenfeld (DJ Qualls): A research assistant with a double major in Physics and Theater, Toby was used by Sheldon to play his fictional first cousin "Leopold Houston" from Denton, Texas, a character Sheldon invents as part of an elaborate lie told to avoid going to see Penny sing. Cousin "Leo" is a recovering drug addict who ends up cuddling with Penny on the couch, much to the chagrin of Leonard. He was also willing to argue with Sheldon on the subjects regarding drug use, showing knowledge on the subject.
Dennis Kim (Austin Lee): Dennis is a 15 year old child prodigy from North Korea whom Dr. Gablehauser hopes to recruit into their Ph.D. program. Dennis and Sheldon have an antagonistic relationship when the former excels Sheldon in every way. The guys make arrangements so Dennis can meet girls of his age and get distracted from his own research. The plan succeeds and Dennis is seen kissing with a girl, no longer an intellectual threat to Sheldon.
Dimitri (Adam Gregor): A former Russian physicist who works as a janitor in the Cal Tech Physics Department after he immigrated to the United States. Sheldon makes an arrangement with Dimitri to be on his Physics Bowl team, since Sheldon cannot enter the competition as an individual, only as a team of four. The arrangement calls on Dimitri not answer a single question, but he does correctly answer a question when Sheldon is unable to answer the question. Sheldon refuses to allow Dimitri's answer to be counted, and ultimately loses the game. The name of the character is never used on the show, only in the credits.
Mike (Andrew Walker): A random boy-friend of Penny's who picks her up for a date when Leonard is trying to explain to her about the time machine he bought on e-Bay in "The Nerdvana Annihilation".
Missy Cooper (Courtney Henggeler): Missy is Sheldon's fraternal twin sister who is intellectually and socially very much unlike him. Confident, tall and attractive, she immediately catches the attention of Leonard, Howard, and Raj. Sheldon realizes that within Missy's eggs lies the potential for another "superior mutation" like him. He temporarily erects himself as a guardian to Missy in order to choose the best mate for her (she rejects Leonard and Howard in favour of Raj, but his mutism derails any chance he might have with her). Missy refers to Sheldon as "Shelly", and despite their differences she loves him and is proud of his accomplishments.
Eric (Travis Schuldt): a boyfriend of Penny's in the episode The Codpiece Topology
Blain (Tyler Olson) and Tom (Mark Hames): A couple of guys Sheldon tries to fix up with Penny in order to help break her video game addiction. In his clumsiness, Sheldon gives Blain the incorrect impression that he is inviting him on a date (with sex) rather than fixing him up on a date with a woman (Penny). Tom is a person whom Sheldon finds in an online dating service who was matched to Penny because he likes the woman to be the sexual seducer.
Octavia (Octavia Spencer): a clerk working at the California Department of Motor Vehicles who hands Sheldon a drivers permit after he refuses to take the test, but instead points out inaccuracies in the test. The name of the character is not used on the show, only in the credits.
Maria (Elena Campbell-Martinez) and Lourdes (Livia Treviño) a couple of janitors working the night shift at Cal Tech, who discover that Sheldon is living there. Sheldon instructs them not to tell anybody about it. The name of the characters is not used on the show, only in the credits.
Ramona Nowitzki (Riki Lindhome) and Kathy O'Brian (Emily Happe): A couple of female graduate students, who both have a crush on Sheldon, and become his 'groupies.' Sheldon exploits them to get free food. Ramona is the first, first meeting Sheldon after he introduced his work to incoming graduate students. She arranges to have dinner with him in his apartment. The next few days she becomes a permanent presence in his life, pressuring him to concentrate on his research and avoid distractions. Sheldon gets tired of her but is unable to dissolve their "relationship". Eventually, Sheldon reaches a breakthrough in his work, and kicks Ramona out when she asks to share credit with Sheldon for the discovery. Kathy comes into Sheldon's life after he kicked Ramona out.
Dr. David Underhill (Michael Trucco): A MacArthur Genius Grant recipient, David is an experimental physicist like Leonard, who is excited to be working with him. With his leather jacket and handsome looks, Penny is shocked to know that David is a scientist and starts dating him. They break up when she finds out he is married.
Alicia (Valerie Azlynn): A young blonde who moves into an apartment above Leonard and Sheldon in the Dead Hooker Juxtaposition. Penny displays a fit of jealousy at this turn of events, thinking that Alicia is supplanting her in the boys' attention. Alicia is an actress, although more successful than Penny, having landed a role on CSI as a (dead) prostitute. Eventually, to Howard's delight, she and Penny get into a catfight. After the catfight, Penny says Alicia is a "dead whore on TV; live one in real life" just before the gang hears Alicia having sex with a CSI producer.
Mikayla (Jodi Lyn O'Keefe): a prostitute in "The Vegas Renormalization" episode whom Leonard and Raj pay to have sex with Howard.
Dr. Crawley (Lewis Black): An entomologist at the University, Dr. Crawley is visited by Sheldon, Howard, and Raj in order to identify the species of a cricket they found. He is agitated at losing his position at the university, his lab, and, as a result of an extended research trip, his wife.
Bethany (Molly Morgan), Sarah (Sarah Buehler), and Skeeter (Andy Mackenzie): Characters who appear in The Gothowitz Deviation episode. Bethany and Sarah are a couple of girls that Wolowitz and Raj meet in a Goth club. They then go to a tattoo parlor, where Skeeter is the tattoo artist. The name "Skeeter" is not used on the show, but appears in the credits.
Professor Laughlin (Oliver Muirhead): Professor Laughlin is a British professor who leads Cal Tech's Stellar Evolution Research Team, which Raj attempts to join in an effort to avoid being deported back to India. Unfortunately, he does not get the job, because an attractive female member of the team turns up at his interview, and when he attempts to cure his selective mutism by drinking some sherry he was offered, he has too much and makes an overtly sexual comment.
Dr. Catherine Millstone (Elizabeth Bogush): Member of Professor Laughlin's Stellar Evolution Research Team whose appearance causes Raj to drink a little too much in order to talk to her. He ends up offending both of them, thus ruining his chances of joining the team.
"Captain Sweatpants" (Ian Scott Randolph) and "Lonely Larry" (Owen Thayer): Two nerds that are sometimes seen at the guys' favorite comic book store. Captain Sweatpants is a middle-aged bald man who wears grey sweatpants and a City of Heroes T-shirt. Lonely Larry wears a brown suit and is extremely thin. They are also friends with Wil Wheaton.
Abby (Danica McKellar) and Martha (Jen Drohan): With Leonard and Howard busy on a double date with Penny and Bernadette, Raj and Sheldon attend a university mixer where they meet Abby and Martha. Abby takes a liking to Raj, while Martha tries to connect with Sheldon. While Raj and Abby end up kissing, Sheldon completely ignores Martha, who even tries to go to bed with him.
Officer Hackett (Julio Oscar Mechoso), a police officer who comes to investigate the burglary in Sheldon and Leonard's apartment in The Bozeman Reaction episode. His name is not mentioned in the episode itself, but does appear in the credits.
Sandy (Yeardley Smith): A bureaucrat who interviewed Sheldon for a menial job in The Einstein Approximation episode. The name "Sandy" was not used on the show, but appeared in the credits.
Glen (Kevin Brief): A mall security guard stationed outside a ball pit in The Einstein Approximation episode. The name "Glen" was not used on the show, but appeared in the credits. Kevin Brief also played a guard in The Beta Test Initiation episode. In that appearance he was only credited as "security guard" without a name.
Venkatesh Koothrappali (Frank Maharajh): Raj's cousin, a lawyer in India, who tries to negotiate a deal for Raj's part of the One Ring prop from Lord of the Rings. He, by his own admission, is a bad negotiator, and does not get Raj even one of the two JetSkis that Raj wanted.
Dr. Elizabeth Plimpton (Judy Greer): A renowned cosmological physicist from Princeton University known for her work in quantum cosmology who comes as a personal guest of Sheldon in The Plimpton Stimulation episode, and winds up having sex with Leonard and Raj, and expresses a desire for a foursome with Leonard, Raj, and Wolowitz.
Joyce Kim (Ally Maki): A previously unseen character who appears in The Staircase Implementation, Joyce Kim was mentioned as a former girlfriend of Leonard; their relationship only lasted a month, after which she defected back to North Korea. However, in The Staircase Implementation, Leonard admits that she was not a girlfriend, but a North Korean spy who attempted to get secrets from him by seducing him.
Joy (Charlotte Newhouse): An exercise-freak woman whom Leonard meets on a blind date set up by Bernadette via the "Girlfriend Pact" with Howard. Although Leonard finds Joy's personality to be repulsive, he agrees to go out with her again based on hints she gives that it will be easy to have sex with her.
Special Agent Angela Paige (Eliza Dushku): an FBI agent investigating Howard Wolowitz's background for a security clearance in The Apology Insufficiency episode. While interviewing the guys, Raj vomits on her shoes, Leonard hits on her, and Sheldon reveals too much information about Howard, and to add insult to injury reveals too much information about Leonard.
Wyatt (Keith Carradine): Penny's father. He comes to visit and Penny feels compelled to involve Leonard in a ruse to make him think that they are still together. (Wyatt reveals that he favors the successful Leonard over Penny's other "loser" boyfriends.) When the plot is revealed, he gets mad at Penny and feigns anger at Leonard. But after Penny leaves the room, he encourages Leonard to keep trying to get Penny back.
Professor Glenn (Rick Fox): A former professor of Bernadette's, whom she reveals to Howard that she dated for a year. At 6'7", dark and handsome, Glenn is physically the polar opposite of Howard, leading the insecure Howard to feel like he'll never measure up (literally as much as metaphorically).
President Siebert (Joshua Malina): The president of the California Institute of Technology; he is married and has a teenage daughter. Siebert sends Sheldon on an expedition to the North Pole at the end of Season 2. In Season 4, he organizes a get-together with wealthy donors and faculty in order to garner funds for the university's programs. He urges the guys to make an appearance to schmooze the donors so that they will give to their projects. Seeing Sheldon's behavior toward the donors, he asks that he not appear again. A recurring gag is that President Siebert bumps into Sheldon when he is using the urinal.
Mrs. Latham (Jessica Walter): A wealthy widowed benefactor who takes a shine to Leonard at a donor/faculty get-together. She asks him out with the implication that he have sex with her in order to get the lab equipment his department needs. He at first resists, but after she says that she is going to fund his project anyway, he relents. Everyone at the University congratulates him for selling himself out for the money.
Emily (Katie Leclerc): A deaf woman whom Penny matchmakes with Raj. She shows interest in Raj when he spends a lot of money on her. Then Raj's parents get the bill, and force him to break up with her. Although Raj resists, he does tell Emily that all the gifts he gave her have to go back. She then loses interest in Raj.
Alice (Courtney Ford): A girl who meets Leonard at the comic book store and then seduces him. This occurs during the period when Leonard was having a long distance relationship with Priya. Leonard initially gives in to temptation, but at the last minute pulls away. Next time he's communicating, via the webcam, with Priya, he confesses his weakness. To his surprise, Priya has no problems with it. Upon questioning, Priya reveals that she has not been faithful to Leonard. They then break up.
Jimmy Speckerman (Lance Barber): a former high-school jock who used to bully Leonard on a regular basis. He later contacts Leonard in "The Speckerman Recurrence", with an idea of inventing glasses that turn all movies into 3D. He is oblivious to the bullying nature of his actions against Leonard, having believed that they were "having fun" and describing him and Leonard as having been "a comedy team".
Siri (Becky O'Donohue): embodies Siri (software), the voice-recognition system used on certain smartphones and on which Raj developed a crush, because of its sexy female voice.
Characters that appeared in only a single scene
These characters appear in a single scene. The list is sorted by chronological order of appearance, considering their first appearance in the show.
Sebastian (Steven Yeun) Sheldon's former roommate who left Sheldon on bad terms. Sebastian makes a single appearance in The Staircase Implementation, where he tells Leonard to "run fast, run far" from Sheldon. His name is not used in the episode, but is listed in the credits.
Louie (Ajgie Kirkland) A former occupant of the apartment in which Penny lives. He is a cross dresser whom Leonard mistakenly runs into in The Staircase Implementation episode, and who refers to Sheldon as "the crazy guy across the hall". The name Louie is used by Sheldon, referring to him/her as Louie/Louise. Sheldon implies that Louie is a cop.
Mrs. Fowler (Annie O'Donnell): Amy Farrah Fowler's mother. She appeared in a webcam conversation with Amy and Sheldon in which they lie to her about the nature of their relationship, telling her that they are in a romantic relationship which includes coitus.
Todd Zarnecki (Christopher Douglas Reed): Appears in "The Zarnecki Incursion" episode as a hacker who "steals" Sheldon's possessions in the World of Warcraft online game. Being physically much larger and more intimidating than the four main characters, they back down from confronting him, but Penny sets Todd straight with a swift kick to the groin.
Angelo (Peter Onorati): A barber who substitutes for Sheldon's regular barber, Mr. D'Onofrio, when Mr. D'Onofrio falls into a coma.