Nancy Claus is a vampire, the wife of Santa Claus, and, three-hundred years prior the events of Billy and Mandy Save Christmas, Nancy was appointed the Head-Head Vampire over all the other head-vampires. She is also the reformed Main antagonist of the episode.
According to her husband, Santa, Nancy was a vampire long before the two of them had even met, but that didn't discourage him from courting her. According to Nancy, Santa having always had, in her own words, a "odd desire to give presents to complete strangers," had shown her the joys of giving, (blood,) not just recieving.
After the two of them got married, they purchased a "Pole of their own". Knowing that that making Christmas every year would be a lot of work, they both agreed to divide the work equaly amongst the two of them. Over the years, however, Santa became more pre-occupied with checking his 'naughty and nice' list and updating it, leaving his wife to pick up his slack. In Nancy's own words, Santa "works one night out of the year," while she is "working her fingers to the bone the other 364".
According to her husband in the Christmas-special, what happened there actually wasn't the first time that Nancy had turned her husband into a vampire--in fact, according to Santa, Nancy had apparently turned him into a vampire at least five or six other times before (as the most recent time was either the sixth or seventh occurrence).