Rokuro doesn't expect Kylo to know, and in an ideal world he won't. He has kept the true cost of his power a secret from his closest comrades, if only because he doesn't want his pain to distract them in battle or weaken their resolve.

He knows it will kill him, and made peace with that fact long ago. All that matters now is surviving long enough to die in the right time and place. To fall any sooner than his master allows would be shameful.

Teeth gritted, he drops the shield with one hand and yanks the bandages from over his right eye with the other. All at once there is light, blue and dazzling, like sun dancing upon the ocean floor. Rokuro says nothing. Words aren't required for this particular power, only sight.

With his chalcedony eye he sees inside of the Sylphid, both of them, past their skin and into their muscles/organs/bone, because he is the master of water, and humans are also water. Born of the sea and eventually returned to it.

"Please, die." Barely a whisper, and in the next second the Sylphid are dropping their too-large weapons and crumbling to their knees, both clutching their chests. Gasping, writhing, and eventually going still.

A moment later, and Rokuro also crumbles to his knees, for related but vastly different reasons. Bleeding from his glowing chalcedony eye and nose. Out cold.