Who is Gran/Djeeta ? as we are all aware, Gran and Djeeta are classified as unknown race, benefiting from all Bahamut weapons.

The Unknown roster currently features demons, half demons, primal beasts, robots, and vampires,

I highly doubt Gran and Djeeta are any of those and definitely unlikely to be artificial life form.

But Primal Beast is definitely a possibility. My theory is that Gran and Djeeta attribute their Unknown genetics from being part Primal Beast. It is known that Their father is a well traveled skyfarer, and was in company of several primal beasts in his party, namely there was a particular female Primal Beast, we know her as Rosetta.

I think it's highly likely that Gran/Djeeta are to fruit between those two.

TLDR: Hypothesis, Gran/Djeeta are part Primal, Rosetta is mom.