In The Big Bang Theory, the best characters show up frequently, embedding themselves in the minds of fans. However, there are dozens of interesting individuals who only appear once throughout the show. These characters either push the narrative forward or serve as a refreshing break from the usual proceedings.

RELATED: Every Season Of The Big Bang Theory, Ranked By IMDb Average

While some of the single-episode characters easily fall off the memory, others leave a lasting impression. This isn't necessarily because they are hilarious but because they are part of great scenes or have better traits and arcs. Fans of the show can't help but wish that these individuals would have returned for more.

David Underhill

Penny flirts with Dr David Underhill in The Big Bang Theory

David begins working with Leonard on a project and outshines him. The Macarthur Grant recipient is also revealed to be a Black Diamond skier and member of a rock band.

The physicist is mostly portrayed as a rival Leonard cannot compete with. Apart from outshining him in science, he also threatens to destroy his chances with Penny. He is the fun bad boy and Penny falls for him because he is more outgoing than the physicists she knows. She says yes to everything David asks yet she denied the same requests when they came from Leonard. Luckily for Leonard, Penny's whirlwind romance with David doesn't last long as she soon finds out he is married.

Josh Wolowitz

Howard meets his brother Josh in The Big Bang Theory

After Howard moves into his later mother's house with Bernadette, Josh shows up claiming to be his half-brother. The revelation breaks Howard's heart as he thought his father never started another family after breaking up with his mother.

Josh mainly serves as a reconciliatory bridge between Howard and his father. Having hated the man all his life, he gets to understand that his old man wasn't so bad. Through Josh, viewers also get to see just how much Howard and his friends live in a bubble as they had never met someone studying oceanography. Josh is also very impressed by Howard's career, qualifying him as one of the best relatives in The Big Bang Theory.

Robert Wolcott

Reclusive physicist Robert Wolcott is angered by his own research in The Big Bang Theory

Dr. Robert Wolcott is Sheldon's pen pal who happens to be a reclusive topologist. Sheldon goes to meet him at his cabin in the mountains so that he can assist him with his string theory research.

RELATED: The Big Bang Theory Characters' 10 Best Career Achievements

Wolcott's eccentricities are what make him a great character. His weird habits include keeping a jar full of his toenails and fertilizing tomatoes with his own poop. His paranoia about people stealing his research is as bizarre as it is comical. Upon realizing how much Wolcott has achieved by avoiding life's distractions, Sheldon gets the temptation to live like him.

Alicia

Penny calls out Alicia for using the guys to do favors for her

Alicia is introduced as Penny, Sheldon, and Leonard's neighbor who lives on the floor above them. She is an aspiring actress like Penny and has a way of getting all the guys to fall for her.

Alicia exists to pump up Penny's level of frustration. Penny is used to getting all the attention from the guys but Alicia steals all that. Penny is also revealed to be a successful TV actress yet Penny has struggled to make her dream of being a movie star a reality. Penny's efforts to take Alicia down lead to what could qualify as one of the biggest arguments in The Big Bang Theory and it's all settled via a cat fight.

Theodore

Theodore prepares dinner in The Big Bang Theory

Theodore temporarily rents Sheldon's room in Apartment 4A. After witnessing Sheldon and Leonard's fight, he advises them to not break off their friendship.

Something needs to be done to save the situations when Sheldon and Leonard find themselves on the path to enmity and Theodore's presence helps with that. He is initially easy to dismiss since he comes off as a creepy old man but he is shown to be very wise. Thanks to him, the two friends are able to reconcile.

Christy Vanderbel

Howard flirts with Christy in The Big Bang Theory

Penny's friend from Nebraska shows up to spend a few days with her and Howard falls madly in love. She is portrayed as promiscuous since she starts seeing Howard and sleeps with Randall while engaged to Penny's cousin. Penny also jokes that Christy has slept with every guy in Omaha.

RELATED: Which Character On The Big Bang Theory Has The Best-Paying Job?

Christy temporarily activates Howard's boyfriend mode since most of his attempts at seducing women fail. Though he has feelings for her, she uses him for material gains as she demands he buys her something every time they go out. Howard's mother isn't a fan of her because of her personality and their exchanges make for some fine comedic moments.

Angela

Angela stares at Raj at a restaurant in The Big Bang Theory

Tired of not having the courage to approach women, Raj takes a drug meant to cure him of his social anxiety. The drug works, resulting in him successfully seducing a girl named Angela at a coffee shop.

Angela and Raj hit it off so well that there is a hint that the two could become one of the best couples in The Big Bang Theory. Unfortunately for Raj, everything about him tends to be on the extreme side. Just as he is always too anxious, he becomes too social to the point where he strips. Consequently, Angela leaves him.

Mrs. Latham

Caltech donor Mrs. Latham flirts with Howard in The Big Bang Theory

During a Caltech fundraiser, Leonard meets a major donor named Mrs. Latham who happens to have a crush on him. Leonard is initially uncomfortable but sleeps with her when she reveals that her donation isn't conditional. She will still give the money whether he rejects her or becomes involved with her.

Mrs. Latham stands out because of her zingers. She claims she has the right to make fun of people because she is rich and she unapologetically does so in a very hilarious manner. Additionally, the donor makes Howard step out of his comfort zone and out of his morals too and this makes him an even better character. It's no surprise that Johnny Galecki was nominated for an Emmy for the episode.

Siri

Raj gives Siri flowers in TBBT

After getting the iPhone 4S, Raj tries to flirt with the computerized voice-command personal assistant, and to his surprise, she flirts back. He then has a dream where he goes on a date with Siri (who has taken the form of an actual woman), but she's unable to talk to him because she isn't real.

RELATED: 10 Things Only Die-Hard Big Bang Theory Fans Know About The Show

Raj's interaction with Siri is also meant to highlight his social awkwardness. It's also used to show that Raj can do things his friends can't. Sheldon is impressed by his ability to successfully flirt with a voice-command personal assistant, referring to it as a "great evolutionary leap."

Trent Monaco

DJ Trent Monaco listens to music from Howard and Raj's band in The Big Bang Theory

When Howard and Raj form a band named "Footprints on the Moon," they get a fan named Trent who keeps commenting on their page. They think he is cool so they stalk him only to be disgusted by his weird habits.

The decision by Howard and Raj to track down Trent is simply their attempt to have a cooler lifestyle. Trent is a DJ with a hot girlfriend so they figure he can teach them his ways. But as people who value hygiene and normalcy, they simply cannot tolerate Trent's behaviors such as picking his nose and swallowing the booger.

NEXT: 10 Hidden Details About Leonard Everyone Missed In The Big Bang Theory