Jack seems to be a disembodied spirit trapped in a dagger. He is able to control people's minds and even take over their bodies, becoming a flesh and blood being for short periods. By causing them to commit terrible, gastly murders, he can become corporeal permanantly. While corporeal, Jack appears to be a proper Victorian-English Gentlemen.
Jack isn't affected by physical attacks while a spirit.. It's not completely sure what happens if he is "killed" in incorporeal form.