The recurring character with the most apperances on The Big Bang Theory is a character we haven't actually seen very much of at all. Carol Ann Susi provides the voice of Mrs. Wolowitz, Howard's mother. Following in the tradition of Cheers' Vera Peterson, Frasier's Maris Crane or Rhoda's doorman Carlton, to name just a few unseen TV characters, we have never properly seen Howard's overbearing Jewish mother. We know she is an extremely overweight woman who has a moustache and may wear a wig and painted-on eyebrows, so maybe we're better off with just the occasional glimpse.
Raj's sister Priya, played by Aarti Mann, has appeared in twelve episodes of The Big Bang Theory. Priya and Leonard dated during the show's fourth season but the relation ended in the fifth season after Priya moved back to India. Raj's parents, Dr. and Mrs. Koothrappali, played by Brian George and Alice Amter respectively, have appeared in nine episodes since the show's first season. So far Raj's parents have only been seen via webcam from India.
John Ross Bowie has appeared as Barry Kripke, a colleague of Leonard and Sheldon at Caltech, in eight episodes. Although Sheldon tried to befriend Barry once to access an open science grid computer and briefly made Barry a part of his replacement social group when Leonard & the gang were hanging with Priya, neither Sheldon nor Leonard seem particularly fond of Barry. Barry has a case of rhotacism which means he pronounces the letters "R" and "L" as "W".
Mary Cooper, played by Laurie Metcalf, is Sheldon's devoutly Christian mother. The character has appeared in four episodes of The Big Bang Theory dating back to its first season. Beverly Hofstadter, played by Christine Baranski, is Leonard's neuroscientist mother. Much like Sheldon, she lacks emotions and is overly analytical. Baranski has guest starred in three episodes so far and is set to appear again during the seventh season.