From just playing the game, regardless of strict lore, this how I feel about carbuncles:

Real Life Carbuncles:


They are inherently loyal.
They will fight for you.
They have their own will.
If you don't have authority/confidence with them, they will not obey you (see: Tataru)
They enjoy food (like the one at Revenant's toll)

The only difference is they been magically created ^^

The fairies indeed seems more intelligent than carbuncles in the game.