In the human world, there was chaos. Lots of sins. Some people tried to spread God's love, others doubted it or just straight denied it.

Reasons why heaven and hell started sending its own to corrupt or make the world better. That's where Guest and Zael appear.

Despite the constant war between Hell and Heaven, some angels and demons fell into the prohibited. So called love or even getting along. That was a complete sin. Resulted on fallen angels and punished demons for eternity, lowering rank.

Humans couldn't see them, just feel and get lead by it. Just dogs could, or babies. Obviously. Despite it, angels and demons had the ability to become "human". Be seen by others and interact. They were limits, of course. Only could be used to lead to good or bad. Never expose identity.

Angels had this scent of heaven, sweet and comforting. Demons didn't. They had sutile, but rotten smell, putrid. Humans could sense it but not quite.