There are many guilds characters can belong to. Here are several examples of organizations that any character can belong to.
Open Air- Explorer’s guild. Sometimes they enlist the aid of X-Kai
The Inspectors- guild of highly trained investigators that answer to no particular crown. They are policed by each country’s government and in turn solve many crimes that were previously unsolvable.
The People- A society of magic users whose power is vast. They typically work in their own groups without the aid of other guilds.
Polite Society- A sect of aristocrats who hunt monsters and demons. Sometimes enlist the aid of X-Kai or the Open Air
X-kaibutsu- Monster Slayers of the world. Card carrying members of this guild are granted a much easier time crossing from country to country. * Nicknamed X-Kai
These guilds are considered to operate outside the law. A member of these guilds tend either not to care about law or go directly against it.
The Shadowed fist- Like the Open Air, they tend to explore. Their main aim in the past was to gain powerful artifacts for their dark masters. Now they seem only to seek treasure and beat the Open Air at their own game.
The guild of Darkness- A guild of top assassins who leave so little clues that even the Inspectors have trouble finding them. It is said that one gains special affinity to darkness when they become a member.
Summoner’s Guild- A guild that seeks to expand its knowledge of magic by summoning beings from the soul flow. They tend to avoid especially dark beings. In some cities, summoning is forbidden and thus they do their “art” without permission. It is rumored that some members of “the people” are also members of the Summoner’s guild.
Manslayer- A guild of mercenaries and murderers whose goals are not yet clear. They leave a trail of bodies in their wake, though many of their victims where those who were said to have commited terrible crimes.
*This is not to say carring weapons around on your person is illegal. There are monsters running about after all, but most toll locations will need to see identification and such.