Jessica, as it turned out, was actually a legend, a popular myth, a story that got around by word of mouth. A scorned lover betrayed by her groom the eve of the wedding made a pact with the devil and took her own life. In the said pact, she was awarded an evil power which she used to haunt weddings and sniff the lives out of all those who are present. 

Kris, as it turns out, reminded Jessica of her former lover and hence her mission to ensure that the wedding would not end happily ever after...at least only for her.