Balance
The scales are all about balance, so let's make it so. Make them Druids who harmonize the forces of nature so that multiple species can exist under the most beneficial conditions. Allow them to act similarly for humans, like "Jedi for judgment" from some of the Star Wars stories: rather than ordering about men with clubs and cells, they propose inspired solutions that allow people or civilizations voluntarily to find peace with each other. Their culture is one of harmony - which is often to say of staying out of each others' way, which is one reason why you find them so much in the wilderness; but they also move among all the other Forms, bringing wisdom and collaboration, and are also known for building great institutions of learning, philosophy and reasoned debate.
Incidentally, I think there may be better ways to describe these creatures than "races". That usage seems dated, recalling the days of H.P. Lovecraft, where it seemed believable to an audience that a small diverged group of people would turn into degenerate fish-creatures. Great stories, to be sure, but writing a new story that way you'd be fighting other assumptions and associations. This is particularly true in this situation, where these Forms seem to depend on the time of birth, and don't breed true -- at least, not unless some folks are very disciplined in following the rhythm method, and even then there could be slip-ups. I'm favoring "Forms" here in a Platonic sense; you could say Archetypes, Resonances, Modes... Or you could go accurately with the technical "Phenotype" (polyphenism), or make it less awkward by altering it to "phene". The term "Caste" in reference to insects is usable.