A wolf in sheep's clothing is someone who tries to trick you into trusting them by presenting themselves as unthreatening, then unexpectedly attacks you.
A poser is someone who pretends to have a "better" social/intellectual/economic position than they really do. @Jay's example is slightly odd (though not uncommon), in that the supposedly "rough" background is in fact considered "better" in certain social contexts (it has more "street cred"). It's an example of inverse snobbery.
It's not clear from the question exactly what kind of "good" image our subject is presenting, what kind of "bad" reality is being concealed, or why she's doing it. Either or neither of the above might apply (but probably not both).
Other possibilities are cheat, liar, deceiver, imposter, pretender, fake, faker, fraud, sham, decoy, defrauder, dodger, double-crosser, double-dealer, trickster, charlatan, two-faced [person].
Personally, I like duplicitous (deceptive in words or action) for general use. But if it's a woman pretending to be sexually modest when I know she's not, I might call her a closet sexpot (a term of my own invention here, but which I'd expect to be understood by anyone I might say it to).
Best Answer
The classic idiom in English is "third wheel". This can be used both against both parties (the couple and the third party). For example:
The couple might say: "He is such a third wheel. I wish he'd stop trying to hang out with us."
The third party might say: "They make me feel like such a third wheel. I don't understand why they invite me along when they clearly don't want to hang out with me."
A more pejorative (and much more vulgar) term might be "cockblock". The implication here is that the intruder is preventing the couple from "getting it on" (having sex) by blocking the male's cock (penis).
"Tommy is such a cockblock. He needs to get a life."
However, cockblock isn't necessarily only about stopping sex. In fact, cockblock has been expanded to include any individual that stops something that people want to do, especially in online gaming:
"Tommy is such a cockblock. I almost got to that extra-ammo pickup, but he killed me before I could. He needs to get a life."