Sara Gilbert stars as Leslie Winkle, a physicist that works at the same Caltech lab as Leonard.  She has had an on-and-off again relationship with Leonard and is bitter enemies with Sheldon.

Leslie Winkle Quotes

I saw your pupils dilate when you looked at her, which, unless you're a heroin addict, points to sexual attraction

Leslie [about Leonard's interest in Penny]

Leslie: Just so we're clear, you understand that me hanging back to practice with you is a pretext for letting you know that I'm sexually available.
Leonard: Really?
Leslie: Yeah. I'm good to go.
Leonard: I thought you weren't interested in me.
Leslie: That was before I saw you handling that beautiful piece of wood between your legs

TBBT Quotes

Stephen Hawking: You made an arithmetic mistake on page two. It was quite a boner.
Sheldon: No, no ... that can't be right. I-I don't make arithmetic mistakes.
Stephen Hawking: Are you saying I do?
Sheldon: Oh, no, no, no, of course not. It's just, I was thinking.... Oh, gosh, golly, I made a boo-boo and I gave it to Stephen Hawking.
Stephen Hawking: Great, another fainter.

Sheldon: Why do you have the Chinese character for "soup" tattooed on your right buttock?
Penny: It's not "soup," it's "courage."
Sheldon: No it isn't. But I suppose it does take courage to demonstrate that kind of commitment to soup.
Penny: How'd you see it? You said you wouldn't look.
Sheldon: Sorry. As I told you, the hero always peeks.