I don't know the history behind the "Ninja" as it exists in FF games - I started playing with FF7; played 8 and 9; and played 13.

But the idea of someone called a "Ninja" being put in a stand-up Tank role is just... wrong.

They ought to be single-target Glass Cannons; extremely-powerful single-target DD, attacking from behind, attacking from "concealement"; applying powerful debuffs - debilitating debuffs like Poison, Hamstring, Shoestring (Shoestring? Yeah... tied your shoelaces together while you weren't looking...)