A classical version in the form of Priya. It really doesn't help that she is highly educated and more of a match for Leonard on an intellectual level.
Zach also takes on this role, although played around with extensively in that he was friendly with the guys and Penny admitted she didn't have much feelings for him.
Weirder Than Usual: In one episode, Sheldon and Amy terminate their friendship, and he tries to hide his loneliness by adopting first one cat, then several more. Leonard tries to express his concern to Raj and Howard, who at first don't understand the problem when he says he's worried about Sheldon. Howard notes that they are always worried about Sheldon, whom they described in an earlier episode as being "one lab accident away from a supervillain." After interacting with him and the first cat, Dr. Robert Oppenheimer, they have a better grasp of the situation.
Howard: Okay, I get it. We're worried about Sheldon.
Wham Episode: Raj and Penny waking up in bed together, which had ramifications that could tear the group apart.