Guide Mentor
Character Role Analysis
Karellen
Karellen is clearly the guide/mentor character, though whether that's a good thing will depend on your reading of the protagonist and antagonist.
Basically, the plot of the story revolves around Karellen guiding humanity down the path to joining the Overmind. He mentors Stormgren on how to form the World State, he ushers in a Golden Age by giving humanity access to some seriously swank technology, and once the children begin to change into the Overmind, he steps in to tell the Earthlings what is going on and how it should be done.
But Obi-Wan Kenobi he is not. Rather than teach directly for the benefit of mankind, Karellen manipulates the people of Earth to suit his own ends, since he is a slave to the Overmind's desires.