Thanks for the advice - looking into it among other options. In the meanwhile, here's a basic background I've worked up for the game.
In 2008, in the aftermath of wars, natural disasters, and the economic crash, fate had one more cruel surprise in store for humanity. On December 5, a team of crack archaeologists uncovered an arch in a tomb near Alexandretta, Turkey dating from the 12th century. Fortunately, one of the team could read the inscription on the top of the arch written in Aramaic. Unfortunately, he read it out loud. This was unfortunate in that word for word, the inscription read, "WHATEVER YOU DO, DON'T READ THIS INSCRIPTION OUT LOUD UNDER ANY CIRCUMSTANCES. WE MEAN IT." Suddenly, the arch was revealed to be a gate, and all the monsters and horrors that had been imagined since the beginning of human storytelling poured out, precipitating the greatest crisis civilization has ever faced.
For years, it seemed as if humanity's extinction was a foregone conclusion. The monsters were bigger, stronger, had powers far beyond human understanding, and were growing ever more numerous. And it seemed that they fed off the flesh or spirit of the humans themselves. However, it turned out that a few things were actually working in the humans' favor. First, the monsters themselves were hardly an organized front. Many of the different kinds of monsters were at odds with each other as often as they were with the humans. For that matter, just because two monsters were of the same kind didn't necessarily meant they got along peacefully. And humanity often produces its best work when it is backed into a corner.
And such was the case in 2012, when plucky scientists Travis Snugel and Tabitha Bonnie developed an airborne mutagen which, while far from solving the problem, radically changed its parameters. The monsters soon found that they could only absorb spirit willingly given - rendering them dependent on caring, affection, and even dare we say, love. Furthermore, the extent of the powers that the monster could access became directly related to the strength of the bond between the monster and the human with which he or she was affiliated. As the relationship grew stronger, so did the monster's powers. The predator-prey relationship had been transformed into a symbiotic one. Scientists quickly termed this the Snugel-Bonnie effect.
Still, the danger remains. The larger more powerful monsters, greater demons, eldritch horrors, nasty tentacled thingies and tunacanes (like a sharknado except while the latter merely leaves you as a messy corpse, the tunacane subsides with the victim smelling like three day old fish - and on the day of Homecoming! Horrors!) developed some sort of resistance to the effect, more or less collecting its prey as harems. However, now the world is defended by monster-human pairs and perhaps, for the first time in years, there is hope. To that end, Doctors Snugel and Bonnie have formed The Monster-Human Institute, a college dedicated to the matching and training of monsters and high-affinity humans.