For those unfamiliar with the term, crossover is when you mix two different things, crossing their "universes" like a character from a game with a anime character's clothes, or appearance, or whatever ~~ aanyway, here are some of the Kingdom Hearts/Final Fantasy crossovers I found ~~>
The best one, I think... >< My favourite
hahahaha, laughed SO MUCH at this one, hehehe
Second Favourite *u* Black Rock Shooter Ventus *--*
Moogles. I'm still not sure what are they for, but they appear in every single Final Fantasy/Kingdom Hearts game. A main caracteristic of them is saying *Kupo* after everything they speak, which is very cute...their function vary, from store owners, pets, companions, even something you can use as a weapon, and as a *useful tool*, like in Final Fantasy XIII-2... in Kingdom hearts, their main purpose is Item Synthesis. they're the ones who can syntethise the Final Weapon, the Save the King and the Save the Queen, for example. In FF VII, Caith Sith rides a giant Moogle, and in FF X, they're used by Lulu as her weapons.
Well, so they are always present, and in each game they look different, too. here are some of them, in some of the games ~~>
These above are from the Crystal Chronicle series... on the order: Ring of Fates, Echoes of Time, My life as a King and the first, original Crystal Chronicles.
Left one: FF VII ~~ Right one: FF Chocobo's Dungeon
Left One: FF Crystal Bearers ~~ Right One: FF VIII