User blog comment:Xilinoc/Ch. 553 - Frozen Cross/@comment-10330822-20131016154408/@comment-11051098-20131017215324

What if the A is simply to show that he's the next Emperor, the next A? Either way, the A could stand above the other letters because Yhwach has it, and Uryû possess it because he's his successor. The other letters could simply define the Sternritter's ability.