I'd push back the Runic Blade ability a good ways, and consider giving them weaker versions of it's effect at earlier stages in the class. Right now, it looks really front loaded with an ability I'd see a ten level PrC getting for it's final power. About the only thing that would keep anyone in their right mind from taking two levels of this class off the bat (10 levels of spell absorption is huge for a level 2 character!) is that it totally disallows multiclassing.
Just from my recollections of FF, I'd make this a PrC. Celes was pretty unique in her status as Runic Knight... and she was a General as well if I'm correct. Even if the numbers don't show her as a high level character off the bat, the whole backstory is that she's risen to that level of power through her career... it's not like you join the army, get genetically engineered day two, and are given a runic sword. She was the creme de le creme of the army.
Smells PrCrific to me.