When a show goes on for as long as The Big Bang Theory, new characters are bound to get introduced along the way. For most of its run, the show kept the core cast of five - Leonard, Sheldon, Penny, Howard, and Raj. In season three, Amy and Bernadette arrived and substantially changed the dynamic of the group and the show.
While the girls became series regulars, some of the new characters only played recurring roles. They came and went, returning sporadically for special occasions. These characters improved the viewing experience with their quirks, humor, and at times inept personalities. They all contributed something valuable to the show, making audiences miss them whenever they weren't on screen.
Denise
Denise makes her debut in the eleventh season episode "The Comet Polarization." Stuart hires her as the assistant manager to the comic book store after writer Neil Geiman visits and puts it on the map. She's knowledgeable in comic books, quickly endearing herself to the group.
Denise develops a relationship with Stuart, finally giving his character a much-needed break. Fans don't really spend too much time with Denise, which is a shame. Her character is the perfect antidote to the negative stereotypes about women and comic books that the show endorsed for much of its run.
Anu
Like Denise, Anu debuts late in the show's run, meaning fans get little time with her. She first appears in the season twelve episode "The Procreation Calculation" and is the woman that V.M. sets up as Raj's future wife. Anu is successful, ambitious, and has a more dominant personality, which bodes well with Raj's passivity.
Her relationship with Raj is short, but the two actually get along. She also becomes friendly with his friends, mostly because, as a hotel concierge, she has connections all over Los Angeles. Anu and Raj break up when he stays in Pasadena instead of going to London with her.
Bert Kibbler
Bert is a geologist working at Caltech. His first appearance is in the season six episode "The Contractual Obligation Implementation." He harbors a crush for Amy for a while, visiting her at her lab and bringing her beautiful shiny rocks.
Bert is incredibly awkward and has problems with social interactions. However, he also excels at his field, even winning the MacArthur Fellowship grant in the tenth season episode "The Geology Elevation." Bert is pretty much a gentle giant to the tee, but his interactions with the group are real stand-outs of the show's later seasons.
Emily Sweeney
Dr. Emily Sweeney is Raj's girlfriend for all of season eight and part of seasons seven and nine. She is a dermatologist with a peculiar and somewhat creepy sense of humor and a fascination for horror and gore. Nonetheless, she accepts Raj with all his quirks, and the two actually seem to have a healthy balance in their relationship.
Emily is arguably Raj's best girlfriend because she genuinely likes his personality and never attempts to change it. She also gets along with most of his friends, although she famously dislikes Penny. She also is one of the few people in the show to stand up to Sheldon.
Wil Wheaton
Wil Wheaton plays a fictionalized version of himself, starting with the season three episode "The Creepy Candy Coating Corollary," which coincidentally is also Bernadette's debut. He serves as an antagonist to Sheldon during season four but eventually becomes his friend, starting with season five.
Within the Big Bang universe, Wil is a tad petty but also supportive. He's also portrayed as somewhat of a has-been and even stars alongside Penny in the sequel to Serial Ape-ist. Eventually, he takes over the mantle of Professor Proton and is even Sheldon's first choice to officiate his wedding to Amy before Mark Hamill becomes available.
Zack Johnson
Zack makes his debut in the season three finale, "The Lunar Excitation," which coincidentally is also Amy's debut. Zack is a dimwitted simpleton who nonetheless shows genuine interest and appreciation for science. He also likes the boys and seems to want to be friends with them.
Zack comes back numerous times during the show's run, appearing in seasons four, seven, nine, ten, eleven, and twelve. He famously asks Leonard to serve as a sperm donor because he wants his child to be smart. Despite appearing in only eleven episodes, Zack is a fan-favorite who fans wished to see more of.
Professor Proton
Television icon Bob Newhart made his debut in the season six episode "The Proton Resurgence." Arthur Jeffries was the host of Leonard and Sheldon's favorite tv show when they were children and a hero to them both. Arthur reveals that he dislikes his Professor Proton fame because it made him a laughingstock in the scientific community and ruined his marriage.
Professor Proton proves to be quite smart and even impresses Sheldon with a paper he wrote. Arthur dies in the seventh season episode "The Proton Transmogrification." However, he continues to appear as a figment of Sheldon's conscience. For his role as Jeffries, Newhart won a much-belated Emmy for Outstanding Guest Actor in a Comedy.
Dr. Beverly Hofstadter
Another television icon, Christine Baranski, had a recurring role in The Big Bang Theory. Starting in season two, Baranski played Dr. Beverly Hofstadter, Leonard's distant and unsympathetic mother. She's a highly respected neuroscientist and psychiatrist who doesn't like displaying emotion.
Cold and analytical, Beverly displays blatant dislike for Leonard while often favoring his two more accomplished siblings. She also has a close relationship with Sheldon, much to Leonard and Amy's chagrin. Beverly eventually mends her relationship with Leonard in season twelve.
Dr. Bernadette Rostenkowski-Wolowitz
As previously mentioned, Bernadette makes her debut in season three. She starts as a waitress working with Penny at The Cheesecake Factory to pay for her studies in Microbiology. She eventually earns her Ph.D. and gets headhunted by a big pharmaceutical company.
Bernadette suffers significant changes in her personality during the show's run. Although initially sweet and docile, she becomes more domineering and rude, especially after her marriage to Howard. Bernadette is arguably the show's most successful character, as she makes more money and holds a higher position than anyone else in the group.
Dr. Amy Farrah Fowler
Amy briefly appears in the closing seconds of season three. Starting with season four, she becomes a full cast member and proves herself as one of the best additions to the show. Amy is a neurobiologist who lived a very sheltered life before meeting Penny.
Like Bernadette, Amy changes significantly from her initial characterization. In later seasons, she becomes more open-minded and experimental. Her friendship with Penny sometimes makes her seem creepy and desperate, but Amy remained one of the show's most popular characters, mostly because of her endearing relationship with Sheldon.