Prismatic units are the jacks-of-all-trades of esper use. Rather than being allied with a particular type of esper and the faction associated with it, they are allied to three of the six types.
Radiant Prismatics are allied with the aspects of Essence, Creation and Law – the light side of esper. They are the self confessed ‘good guys’ – most of the time.
Conversely, Void Prismatics are allied to Chaos, Corruption and Entropy esper – the negative side of esper. Maybe they’re just misunderstood, but they do seem to be linked to the disappearance of whole planets, which is never a good thing.