Amy started on the show during the season 3 finale, where she meets Sheldon. Raj and Howard found her on a dating site and figured that this neurobiologist would be a great fit for Sheldon.
The two date until becoming engaged (with a short break in between) in season 10. Amy is also in Penny’s posse with Penny and Bernadette.
Sara Gilbert as Leslie Winkle – Then
Leslie works at Caltech with Leonard as a physicist. Leslie is a female carbon copy of Leonard, even dressing similarly in hoodie sweatshirts and black-framed glasses. In season 1, Leonard asked her out instead of Penny because he felt like Penny was too good of a catch for him.
Sheldon disapproves of their relationship, causing ongoing tension between the two. Ultimately, Leslie and Howard form a casual, no-strings-attached relationship, with Howard having emotional feelings towards her.
Sara Gilbert as Leslie Winkle – Now
Sara was and still is best known for her long-time role as Darlene Conner on "Roseanne." The actress now has 3 children and is married to Linda Perry.
"The Conners," a spinoff of "Roseanne," aired in 2018. The "Roseanne" reboot was canceled following a controversial tweet by star and executive producer Roseanne Barr.
Mayim Bialik as Amy Fowler – Now
Mayim started acting at age 12, eventually landing her most famed role as Amy in TBBT. She is a neuroscientist in real life, so playing the role wasn’t much of a change.
When the actress is signed on for the 12th season, she received a raise and now earning $500,000 per episode. She now stars as the titular character in another sitcom titled "Call Me Kat."
Simon Helberg as Howard Wolowitz – Then
Simon plays the role of Howard, who is an aerospace engineer and astronaut who has made impressive scientific gains. Howard’s mother is very overprotective.
At the start of the show, Howard hangs with everyone at Leonard and Sheldon’s apartment. During season 3, he meets Bernadette, who worked at the time at The Cheesecake Factory with Penny and was pursuing her Ph.D. in microbiology. Howard and Bernadette make it official in season 5 and have two kids — Halley and Neil.