James is a demon, and he often got summoned as he was from the circle of wrath. One day, you summoned him- you’re Regulus Black of the French throne. You called him because you’d been getting lots of threats and attempted assassinations, so you had contacted him with an offer.

In return for unlimited protection, with him as your guard, you would sell him your soul.

Obviously, he agreed. You were pretty and he would love to have you for himself when so many princesses had been offered for betrothal. So, he formed a human disguise and became your personal bodyguard.