This is not to be confused with a Persona setting, though the two are quite similar.
Shin Megami Tensei (SMT) setting
This is basically a during-apocalyptic and post-apocalyptic scenario. The during part would be the selection of the characters involved to become capable of communicating with the forces that are attacking the human world. From three places, humans are attacked: the worlds of Light, Ether, and Shadow. The three worlds, once cast away from the plane of the human's earth by magic, have returned as a result of the magic spell's time limit finally expiring. Little portals of probing 'demons' are sometimes found, swirling masses of light that indicate from which of those three worlds the intruder has entered Earth from: dark purple for Shadow, a light green for Ether, and nearly blinding yellow for Light. The true reason for the attacks is unknown, but as more humans are swayed to one side or another, those from the other worlds begin to establish their own territories, despite being vulnerable to human weapons.
Small towns and villages are the first to be consumed, the occupants converted to the sides of those that attacked. These unusual beings do not often kill humans, save to gain power and to protect themselves. Instead, the beings attempt to convert humans to their side, to 'befriend' the humans. The beings feed off of feelings, emotions, from humans, absorbing from mere proximity. Some of these beings may go too far, even by their own self-prescribed rules, and kill the humans in their attempts for power to varying degrees of success. Larger population centers, and those with military bases, tend to be safe for the time being, but after two years, much of the world has changed. Governmental influence has waned as a result of many of the roads becoming perilous. Even the skies are unsafe, air travel grinding to a halt as more and more otherworldly airborne beings become common.
The various military forces take control over their immediate surroundings, declaring martial law in many communities. The larger communities with only the police to help keep the residents safe, are beginning to try an unusual tactic... Negotiating. Amongst the various clans of otherworldly beings, there are leaders that have emerged from their worlds to take control over their territory. Working with these clans, attempting to placate them before they overrun the human cities, has led to a number of cities learning to at least attempt this. The military towns frown upon this, the leaders of those places trying to keep their homes protected while also attempting to gain the resources from bigger cities.Where the characters come in
Basically, the characters are most likely thrust from a world like the current Earth into this unusual one. Commerce and industry is changed forever as a result, the flow of goods falling to a trickle. Truckers are the safest when it comes to ferrying goods around the various countries and to the different towns and cities. Trade routes along the highways are created, with checkpoints made to make sure that none of the 'beings' from other worlds are interfering. These checkpoints are either military or police-controlled. The majority of the world's population continues to grow as normal, though there is a major issue with food shortages in many non-coastal areas and where crops do not grow well.
The characters have many options: they can become negotiators, by chance or on purpose, but there is no formal training given the fact that the otherworldly beings are all quite different from humans and few humans have ever had a chance to study them enough to learn what works with them. They could be a part of the military or police force, which are recruiting en masse to try and bolster their strength and defend more towns, cities, and trading routes. They could simply be curious teenagers, interested in the (as many churches call them) 'demons' that have invaded the world. The churches have even begun arming their holy ones in an effort to protect their faith and worship, some going as far as to send out hunters to try and end the 'demon' threats.The 'Enemy'
If you are familiar with the SMT series, or the Persona series, then you might have an idea as to what I'm going for here. The 'demons' are actually mythological beings of various temperaments. Those from the world of Light are considered to be 'holy', or a force of good. Those from the world of Ether are more neutral and somewhat self-serving in comparison. Those from the world of Shadow are most often depicted as evil by many who meet with them. Many of these clans actually take the appearance of various beings from myth, an equivalent of their own people from their worlds. Harpies, fairies, deities (though not actual deities, they take on the appearance that myth describes them as), and still others. If you're interested in exactly what demons are possible, here is a minor list, including their 'clans' that the various races belong to: http://megamitensei.wikia.com/wiki/List_of_Shin_Megami_Tensei_Demons
Feel free to make a character from the enemy- they may also be negotiators, scouts, harvesters (those who search for food and humans to gain power from), as well as who knows what else. These are just suggestions and ideas, after all.
One thing to keep in mind is that as the demon gains strength, both from learning and absorbing power from the feelings and emotions of humans, there is a chance that the demon may evolve somehow. This is something that no human has ever witnessed yet, and may not be available for all demons of any clan. How the demons gain strength is a peculiar process- typically, just by being near humans, a demon gain strength. It is when a demon that is a 'match' for a human that they gain large amounts of energy however. A 'match' is when their personalities compliment one another, and are much the same. So far, there have been very few 'matches' in this regard. Beyond gaining energy, the 'demon' is much like a human, with their own feelings, wants, likes, and dislikes. And they are as individual as humans are, making negotiations tricky for the most part.
Humans that live in towns that have been taken over are used in various manners- as servants, workers, or perhaps are simply lived with, cohabitating between the denizens of the two different worlds. Most demons of the Light and Shadow worlds cannot stand one another, though those of the Ether are capable of getting along with just about anyone and those clans are often more successful in working with humans than the other two.Binding
In some instances, a 'demon' and a human will enter into a sort of contract or agreement with one another. Doing this requires the demon's energy to filter with the human's, and vice versa. In this manner, the demon gains additional energy as well as receiving more than normal from being near that particular human. The result of this may sometimes allow the human access to magical energies like the demon is capable of utilizing (for example, binding with an Undine would likely allow the human to use ice-magic). Only one demon may bind with one human at any time... but that doesn't mean that other demons cannot attempt it. When a second demon binds to the human, the energies of the three combine, and the two demons 'fuse' into a new one, a form of evolution in some cases, while in others may result in a demon of a completely different clan.
Finally, in rare cases, a binding can do one of two other things. It may evolve the demon right then and there, depending on if the demon and human are a match or not, as well as the amount of time spent around one another (the amount of time that a demon spends around a particular human also determines the amount of energy gained from the human's emotions). The other is an unconfirmed rumor that the human may either become a demon, or be able to summon a Persona (if you know what those are, you'll probably understand what I mean).In Closing
If this sounds interesting, let me know. I'm also unsure if we need any sort of system for this, thought perhaps some sort of 'light' system would be best to describe the relation of powers between the different 'demons'. Post your thoughts, worries, or interest here, and let's see if we can get something going. Sexual RP is possible, of course, depending on what you guys might enjoy in such a setting.