Mandy Chao is a marine biologist at Caltech.
In "The Matrimonial Momentum" (S9E1), Leonard almost blew his wedding day when he admits to kissing a woman while on his North Sea expedition. There had been a lot of drinking, he'd been seasick and she was a smoker so it wasn't even that good a make-out session. He also chipped her tooth. Penny says that she thinks she can get by it since they weren't engaged at the time though she is not happy about it. Later in their honeymoon hotel room, Penny can't stop thinking about her and she asks him if he feels guilty. Leonard admits he feels guilty every time he sees her at work. What? Their honeymoon ends then and they go home arguing and slamming the doors of their respective apartments each spending their wedding night alone. Sheldon also knows the girl and calls the physicist both beautiful and brilliant. First he think about dating her since Amy broke up with him, then he feels her standards are too low since she likes Leonard.
She appears in person in "The Separation Oscillation (S9E2), where Leonard wants Penny to meet her so he approaches her at work. The kiss appears to have barely made an impression on her and she seems to sleep around. Leonard tells her about Penny's concerns and she even makes Penny comment that it seemed like he was trying to sabotage his relationship. She gets uncomfortable as he starts to analyze himself and decides that he may be hurting their marriage because he still doesn't feel worthy of Penny. Using Mandy as an drafted therapist he mentions a dream where he was nursing a baby in a cave. She then states that she can't believe Penny was worried about her.
Melissa Tang is the actress who played her.