According to Robert Place's brief notes in the companion book, Le Fanu's style 'introduces psychological insights that were innovative at the time,' and so the card is said to represent someone who has insight into the unconscious and people's hidden motives. In other words, a sensitive type.It must have been pretty tough to come up with a character from Dracula or vampire legend that would fill the bill of Knight of Cups. I guess this link, tenuous though it may be, will serve.
I myself will always associate Knight of Cups with a particular character from the film American Beauty, the boy called Ricky, who spends his time mooning about on the sidelines of life, filming plastic shopping bags floating about on the breeze, and proclaiming life to be so beautiful that sometimes he feels like he 'can't take it'. I've provided a link many times to the clip in question, but I find it so perfect for the Knight of Cups that I can't resist showing it again:
Maybe I've drawn this card today to remind me to try not to be such a cynical butt hole. ha ha ha ha Seriously, though. Sometimes maybe seeing fathomless beauty in a bit of trash blowing around might make a nice change. (And with the level of litter picking our council provides, may as well try to get something positive out of all those blowy crisp packets, eh?)