Amy Farrah Fowler

Amy Farrah Fowler is a neurobiologist who happens to be Sheldon's friend and a girl, but not a girlfriend.  She met Sheldon through an online dating site after Howard had covertly set up an account for Sheldon.

Besides being in many ways a female version of Sheldon, one of the funny things about Amy is her quasi-sexual comments she makes to Penny.  So, although similar to Sheldon in many ways, a big difference is that while Sheldon appears completely A-Sexual, Amy appears bisexual, if not in actual action, at least verbally.

Amy's research deals a lot with primates and she's even taught a monkey to smoke.  And hey, smoking monkeys are just plain hilarious.

(Amy Farrah Fowler is played by Mayim Bialik.)