In Season 2, Leonard, Howard, and Raj instantly become smitten when a beautiful actress called Alicia moves into the building. Although the guys also fawned over Penny when they first met, she didn't lead them on or take advantage of them. (usually.)
Alicia, on the other hand, flirts with the gang to encourage them to paint her bedroom, move her stuff, and set up her satellite dish. Penny tries to warn the guys that Alicia is using them but they are too blinded by lust to listen.
At first, Penny worries she might be acting unreasonably jealous. But when she asks Penny how much physicists earn, she realises Alicia will only date the boys if they have money. When Penny politely asks her not to take advantage of the guys, their quarrel descends into a full-on catfight.
Even though there've been many blatantly manipulative characters in the show, Alicia's behaviour is more subtle. However, this makes her more detestable since she comes across as genuinely nice person when we first meet her. She's so coy, it's not evident how duplicitous she is until she has the guys in the palm of her hand.