As Lafitte understands it, Sandra Fuentes was a professional sea captain who owned her own ship and lived off of charter fishing and tours who also happened to have a striking resemblance to his long dead lover Catherine Sallier. They met a few years ago and began to spend a lot of time with her as he marvelled in the similarities between the two women. Then, he made a terrible, terrible mistake. Gripped by a passion and an urge he had never felt before, he Embraced her. Since then, he has been living in a kind of hell, hated by the woman he loves, whom he's changed into something she despises, and with whom every attempt to explain or make up for the mistake he made is rejected.
The truth is that Sandra Fuentes was a drug-addicted prostitute who happened to be in the wrong place at the wrong time when Simon Castle was looking for a subject for a scheme he'd conceived. She was lured to the hotel with money and drugs and, there, subjected to torture and the flesh-warping powers of vicissitude to make her match the portrait provided by Paul Delacroix of the corsair's former lover. The psychological torment they put her through pushed her into catatonia, at which point she was sent (via Berenger's connections) to New Orleans for the Lasombra bishop to finish the job. She was given a new history, new memories, and a new personality which Paul knew would appeal to Lafitte. Hooks were deeply implanted to allow him access to her mind so that she could report anything she learned and take orders without remembering them. She was made into a Manchurian Candidate.
When she was returned to Galveston, she was provided with a boat, an apartment, and all of the papers, identification, and records she needed to be a real person, all provided by the Castille's and their numerous contacts within the city. Her meeting with Lafitte was arranged and she was carefully instructed which buttons to push to gain his affections by the master manipulators. With the help of a Voodoo charm provided by the New Orleans Sabbat, Lafitte was half-forced, half-tricked into Embracing her. And then the real manipulation began. She was conditioned to be horrified by her new state and to despise her new sire. Already blood bound to Delacroix, nothing he could do would force her into his affections (to his credit, he didn't even try). Further, having not been given permission to Embrace her, he's had to keep her existence a secret from the Prince in Houston. The strain on Lafitte has been incredible and could, conceiveable, break him, if played correctly.
Sandra, through all this, has no clue that she is not who she believes herself to be. She makes her reports and takes her orders with no conscious memory of the events and nothing has occured to convince her she is not who she claims to be. She has no recollection at all of her previous life and her only remaining relative (an aging mother) has been taken care of by the Castilles to insure that nothing goes wrong with their ruse. Currently, she is serving as second mate on Lafitte's own ship, because he can't afford not to keep her close, but the tension is effecting the both of them and the crew is beginning to talk.
Sandra is a passionate and independant woman with a fierce temper and a love of life, which is part of what made her transition so painful. The other major part is that she is a staunch Catholic and is unable to not believe that she is now damned in the eyes of God and beyond redemption. Rather than be broken by this, however, she's determined to find some solution, either a means to become human again or a means to redeem herself. This is part of her conditioning, as Delacroix intends to trigger in her the idea that she will become human again if she kills Lafitte as soon as he's ready of the timing.
As noted, she hates Lafitte, but she also truly loves him, which damns the situation even further and makes her unable to simply cut loose and try and find her answers on her own. It is also apparent to him, which twists the knife in his gut a bit more whenever they are near each other (which is quite often). Over the last couple years, her anger has taken a less overt form and she's just become very cold towards him.
She's distant from the rest of his crew, since they are also (she believes) damned and because they are his, first and foremost. This makes her very lonely when they are at sea, though she weathers it very well and you'd not know to look at her. Still, when she's in port, she seeks out old friends (most of whom are actually spies of the Castilles and Delacroix) and attends a lot of church.