Xavier grew up with a curious mind. He knew an old lady not far from where he grew up who did a little fortune-telling, herbal cures and the like. People would come to put curses on people, or take them off. Xavier was pretty sure most of it was coincidence, and he confronted her, asking if she really believed what she was doing. She just arched an eyebrow and smiled.
He spent his school years looking for more answers into the way the world worked, and ended up studying biochemistry and physics for seven years. None of the answers go found there were any more satisfying than that old woman's enigmatic smile.
So he turned to voodoo, and discovered that he had a talent for it. He became well known in New Orleans as a kind of voodoo therapist, and attracted interesting freinds for it.
One of those was Cassidy, an Irishman who turned out to be a vampire. They got along fantastically, and Xavier considered Cassidy to be closer than a brother to him. Until Cassidy seduced the woman Xavier loved. This was completely unexpected, and Xavier cut off nearly all contact with Cass after that.
Several years later, Cassidy returned to New Orleans, asking a favor. Not for him, but for his friend Jesse Custer. It seemed Jesse had a supernatural entity in his head that he needed to talk to, and Cass figured voodoo would be a good way to go. Xavier, perhaps out of some remnant of love for his old friend, agreed.
It ended in tragedy. Cassidy's return to New Orleans got the attention of Les Enfents du Sang, a group of vampire wannabes who were looking to avenge an old insult. The ensuing gunfight during Jesse's ritual resulted in the death of Xavier's girlfriend, Janis.
Before they left, Xavier made a special point of warning Jesse's girlfriend, Tulip, about Cassidy. He said that Cass wasn't evil necessarily. Just utterly thoughtless. He lived life without a sense of consequence, and that always ended up being trouble. Xavier knew that Jesse and Cassidy were too close for the warning to take with Jesse, but he suspected that Tulip might be willing to listen.
He was right on both counts.