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 provided 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. Carol Ann Susi passed away in November 2014 and the character of Mrs. Wolowitz was subsequently written out of the show.
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 when Priya moved back to India. Raj's parents, Dr. and Mrs. Koothrappali, played by Brian George and Alice Amter respectively, have appeared in the show since the first season. Dr. Koothrappali visited Pasadena in December 2014, marking the first time the character has been seen outside a computer screen.
John Ross Bowie has appeared as Barry Kripke, a colleague of Leonard and Sheldon at Caltech, in fourteen 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 seven 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. Christine Baranski has guest starred in six episodes of the show.