I've only actually played the first game, but as I understand it, NiGHTS is a high-ranking Nightmaren just like the monstrous bosses they fight. NiGHTS happens to be powerful enough to be independent. Whenever the other Nightmaren mug a dreaming visitor, if the visitor keeps hold of one of their Ideyas, NiGHTS finds it more fun to help the visitor wreck everything instead.

Edit: corrected NiGHTS to they/them pronouns (and while I'm here, you can still get the Steam version of NiGHTS for free)