The yoko is a rare fox monster that specializes in magic, particularly elemental fire. In Japanese folklore, yoko are akin to gods, usually serving as messengers and servants to deities when not taking on the role themselves. In those stories, they are invariably powerful, intelligent creatures, many of whom bear a mischievous streak, and this much is usually true.
But all that stuff about serving gods and such is all nonsense, of course. Some yoko probably thought it up as a way to mooch off of the humans by pretending to represent a god, and then took all the offerings for itself. After the first one thought it up, it was only a matter of time until the technique spread, granting it 'common folklore' status among humans, and I'm sure there were a few gutsy yoko who took to claiming to be gods themselves. Anyway, over time the humans paid less and less attention to religion in general and the yoko in particular, probably because the ones serving and feeding the greedy foxes noticed that their fortunes were usually much worse than their more skeptical peers, rather than the reverse.
Within the last few centuries humans' attitudes toward yoko have shifted from servile to a dismissal of the species as myth, and it's generally observed that the yoko as a race are taking it very poorly, blaming humanity for losing touch with its spirituality and thus depriving them of an easy living. Those few humans that still recognize and deal with yoko - mostly monks and priestesses - usually treat them as nuisances rather than holy spirits, and most devil hunters treat them with the same ruthless prejudice as they do demons.
Yoko are somewhat combat-shy, being generally lazy, sneaky, and having no warrior heritage or pack mentality in their culture. Although they're quick to resort to threats and painful demonstrations to get their way, actual violence is reserved for when they face no chance of losing or feel they have no choice. When they do fight, however, they are extremely destructive, and will bring an ungodly amount of power against their target in the form of fire magic. Although their flames can reduce non-resistant enemies to ashes in a single hit, their fondness for tricks and bluffs means they're probably short on actual combat experience. Good luck dodging!"