Oh, look. It's a big snikkedy snake. Water dragon. Slurp.
Leviathan is basically there to illustrate how islands are supported by their Primal Beasts, and I actually really don't have much to say about it? I've aways liked the ways its wings/fins are drawn, I guess. Really my whole memory of the initial story chapters in the Auguste Isles is a blank. I remember meeting Eugen and getting into a fight with this Yu-Gi-Oh card lookin' snake and that's it.
Even its Omega form (the powered up version, basically) doesn't really add much, except for that aqua ring that's hell to fuck around with in the Impossible raid.
Poor Leviathan. Hopefully the other Cygames gachas have treated you better.
Final Score: 2/5 Slippery Snakes