Barry Kripke Ph.D. is a Caltech plasma-physicist-turned-string-theorist and Leonard and Sheldon’s colleague. He is impressively smart and has Rhotacism, which is also kind of cute, but he can be obnoxious, knowing exactly how to tick people off.
Nonetheless, he is quite talented and therefore appreciated (even if not entirely liked) by Sheldon, Howard, Leonard, and Raj. He first appeared in the show during season 2 and has kept appearing in later seasons as well.
Courtney Henggeler as Missy Cooper – Then
Missy is Sheldon’s fraternal twin sister, who is completely different than her brother. While she is very charming and amicable, Sheldon is anything but. She is introduced in the first season, with Leonard, Howard, and Raj all crushing on her.
The men fall over each other, trying to win her attention, and Sheldon tells them that he will choose who gets to date her. Missy finds an ally with Penny, and while they are hanging out at her apartment, each of the guys tries their luck at asking her out, but none of them are successful.
Courtney Henggeler as Missy Cooper – Now
"The Big Bang Theory" was one of Courtney’s first acting roles. Since the gig, she has acted in a few other — mostly minor — roles.
In 2013-2015, she acted as Claudia in the hit CBS sitcom "Mom," where she acted alongside Anna Faris and Allison Janney. In 2018, she became a series regular as Amanda LaRusso in the Netflix hit "Cobra Kai."
John Ross Bowie – Now
John is most well known for his part on TBBT, but he has made himself well known even outside of the series.
He is a regular sketch comedy performer and is very present in the world of comedy, both writing and performing sketches. He has also acted in several films such as "He’s Just Not That Into You" and "What the Bleep Do We Know?"
Wil Wheaton as Wil Wheaton – Then
Wil Wheaton plays himself in the TBBT from seasons 1 to 11. A little acting was required, as he had to play a darker version of himself.
For some time, he had some turf with Sheldon, but the two reconciled in season 5 and became good friends.