If I can just say something about this (not trying to stamp on your hopes and dreams, just using it as a jumping off point). NiGHTS is prime Assist Trophy material, but not a potential playable character. Why? NiGHTS can fly, freely; like, totally freely (yes, albeit limited to a 2D space). His movepool is super limited, too.
I can imagine a NiGHTS assist trophy though, easily.
When NiGHTS shows up, he does his little 'dualise' dance (with accompanying tune, people who've played NiGHTS know what I'm talking about), then starts flying right. When he flies off the right side of the screen, he loops back around and comes out the left. Whenever NiGHTS gets close to a fighter (except the summoner and those on their team, if any). he will try to paraloop them. Getting paralooped is an instant KO. If you get caught directly by NiGHTS, NiGHTS will instead grab you and launch you, recoiling off at an angle. After a recoil, NiGHTS will eventually come back, and will continue flying around until his time limit is up.
