"The Big Bang Theory's" additions of Mayim Bialik’s Amy and Melissa Rauch’s Bernadette (first as guest stars in Season 3, then as full-fledged series regulars in Season 4) were essential to the extending the lifespan of the CBS sitcom, whose previous gender imbalance limited storytelling opportunities.

Bialik would go on to bank four consecutive Emmy nominations for her turn as Sheldon’s extremely patient girlfriend (and later, wife). And we have Rauch’s Bernie to thank for Howard’s transformation away from the walking sexual harassment lawsuit Simon Helberg embodied in the first three seasons.

https://tvline.com/lists/best-tv-characters-late-cast-additions-seinfeld-buffy-the-office/best-tv-characters-late-cast-additions-amy-bernadette-the-big-bang-theory/