A person is dealing with someone who they know has uncanny powers of charm. These can be normal features of The Casanova, or special Mind Control abilities. So, knowing this, a person will be on their guard, right?
Well, not exactly. Because while this guy might be a selfish manipulator of women most of the time, when it's with me, he's different. He's sweet and kind and caring, and everything that other people say about him is because they're just jealous and don't really know him.
Cynical sounding write-up aside, when this trope happens to the protagonist in a story, they'll almost always be right, simply out of the genre convention that The Hero is rarely ever allowed to be wrong. Of course, misunderstandings implying that The Casanova was really a Jerkass all along are still quite common in order for the couple to earn their happy ending.
This trope can also be played from a completely tragic angle, where it's obvious to everyone that one half of this "relationship" is being played like a fiddle, but she refuses to listen to anyone about it because true love is the best thing ever. Changing status quo in this situation usually requires a Batman Gambit by a third party, a What an Idiot! moment by the seducer, or a completely random coincidence exposing him for what he really is. (Alternately, it can played as tragic for the seducer: the former Casanova really has changed their ways, but no one believes them and no one finds out - at least, not until after it's too late...)
Use of this trope is necessary in most stories with a central focus on All Girls Want Bad Boys.
- Anatolia Story: Yuri's and Kail's relationship, as even though Kail has been known to sleep around with lots and lots of women, he falls for Yuri hard and thereafter only has eyes for her. Played with in that it isn't Yuri who makes the claim, but one of Kail's former lovers on her behalf. When Yuri admits that she and Kail sleep together (that is to say, sleep in the same bed; at this point they weren't having sex), the woman responds that Kail has never actually slept with any of his lovers: they'd have sex and he'd go back to his own room. The fact that he's willing to show that vulnerability to Yuri she takes as proof that Kail's love for Yuri goes beyond any of his previous relationships.
- The whole point of Love in the Afternoon.
- Forever Amber takes an odd view of this trope. When Amber first meets Bruce Carlton at sixteen years of age, she's convinced that they're true lovers even after he leaves, and this continues when they reunite some years later. The thing is, Amber knows he's The Casanova. When her stepdaughter becomes infatuated with him, Amber is cruel and blunt in explaining about how Carlton only cares about her to the extent it gets him laid. And yet never at any point does Amber consider that her infatuation with him is any different, willfully ignoring all evidence to the contrary.
- It's this aspect of her character that ends the story when two nobles at the English Court who are normally enemies decide that Amber's highly fickle political whims make her too dangerous to keep around. They forge a note from Carlton saying he loves her and wants her to follow him back to America- and poof! Problem solved!
- Arguably the point of The Twilight Saga. Bella falls in love with Edward, even though he (and her friends) keep reminding her just how dangerous he is. It is even mentioned that the reason Edward is so charming and gorgeous is to lure his prey, since he is a vampire.
- One episode of Maverick has this happen to the daughter of an old family friend of Bret's. Notable in that it's unclear whether the old man's daughter is being manipulated until halfway through, when the man (played by Clint Eastwood) visits his other girlfriend and they discuss how his plans to seduce the old man's daughter and gain access to the ranch are going. Also notable in that the plot isn't resolved by Maverick seducing the daughter, like you'd expect. He actually tricks Clint Eastwood into thinking Maverick's a high-grade gunslinger who could conceivably beat him in a duel, exposing him as a coward when he flees town.
- Too common to be believed on The Bachelor and its spin-offs. Every contestant has something special with the lead in their own mind, only to be stunned when they don't get a rose.
- Thrill Me has this trope, down to Nathan actually saying the line, "I thought we had something different!" Richard attempts to pretend that this is only due to Nathan's Selective Obliviousness, but there are a few moments of Aww, Look! They Really Do Love Each Other that prove that's not entirely true. Not that it's the best relationship regardless.