Perhaps what you desire comes in skirts and chenille top. Perhaps it comes licking an ice cream cone. Perhaps it's fresh, or perhaps it's wizened. Perhaps it isn't even human. Do you wish, fervently, with all your heart, for something good? Do you lick your lips in your sleep while dreaming about something sweet? You desire. You want, as all humans do. Without that craving, your deadened senses drag you down into despair. Desire keeps the wits sharp. Desire drives men to madness. Beauty, power, wealth, sex, desire has it all, is it all, can whisk it all away.
Then and now, it's everything you've ever wanted. Desire knows the number on your bank card, reads your book over your shoulder, and whispers in your ear when you think about buying, about looking up and meeting someone's eye. Since the world began, it's been the voice of temptation. The serpent told her what she would gain by eating, but who made Eve want the apple? Say yes.
Details: Sometimes male, sometimes female, and very often both, or neither. Desire is fluid with its pronouns. It is known to be the most manipulative and cruel of all its siblings.
Desire evicted the god that had been previously possessing Louis, identifying him as a rare, ready-made vessel who will likely be occupied by one supernatural entity or another for the rest of his life. Desire can take over completely, though this is rare. It currently resides in Louis for its own amusement, and to get a taste of mortal life, as it so rarely does.
Desire's connection with Louis is fed by the energy of desire itself. Thus Louis can sense the desires of others, and is compelled to meet them by transforming into whatever it is they want most. This typically results in a physical transformation into their idealized object of sexual desire, but can also lead down more obscure pathways - someone who wants more than anything in the world to be healthy may be offered a temporary reprieve in the form of a miraculous drug, etc.
The benefits of any 'gift' Louis offers in this state do not last, however. When he leaves the person's presence, their effects fade with distance. Any money, for example, would disappear after Louis was outside its presence for more than a few minutes. Whether Desire can create something more long-lasting has not been tested, but since it is in the business of wanting rather than satisfaction, this limited-time offer is likely very much on purpose.
Desire has also created a new shell for itself, something innately human, but also a fresh opportunity to present itself to the world - and its siblings. Being pretty and tailored does get boring.