Melissa Rauch as Bernadette Rostenkowski-Wolowitz, PhD (recurring season 3, starring since season 4) – a young woman with a very high-pitched voice who is initially a waitress co-worker of Penny's, paying her way through graduate school, studying microbiology. Bernadette is of Polish origin and was raised in a Catholic household. Her father Mike (Casey Sander) is a toughened retired police officer and she has four other siblings. Bernadette is introduced to Howard by Penny as a result of an agreement between Howard and Leonard that if either ever ended up with a hot girlfriend, he would get the girlfriend to introduce the other to "a hot chick". At first, they do not get along, apparently having nothing in common, until they find out that they both have overbearing mothers. During season 3, they date and then break up off-screen. They get back together and get engaged in season 4 and marry at the end of season 5. After successfully defending her doctoral thesis, she lands a high-paying research position with Zangen at the end of season 4 (her high income becomes a source of conflict between her and Howard in later seasons). Although generally a sweet and good natured person, Bernadette has a very short temper and can be vindictive and lash out when provoked, often sounding like Howard's mother, and intimidating her colleagues. She is also shown to have a competitive streak.