I think it's fair to put Ignis on the multi-character category because it's possible to use some of her moves while Relius is "busy". Although most of Ignis' moves are actually Relius' attacks (which would make him similar to Litchi) some aren't, and it's also possible to move Igniz far away for a short while.

Also I think Platinum fits much more into the "Character mechanic changing" category than the Projectile one since only 2 of her 6 itens are projectiles. Platinum is a character whose greatest strength is randomly determined by the itens she gets (Guard breaking with Hammer, Reversals with Bat, Spacing with cat-hammer...).

And yes, although the characters in GG are very unique, their peculiarities are not as set as stone as in BB, with a few exceptions.