The Kritenya are my idea of this world's spirit guide. The ancient text for the heroes of the world would not mention the Kritenya. The premise is that they would function like guardian angels.
The crystalline appearance is inspired by the idea of an ethereal being like an angel. Since the Kritenya are supposed to resemble angels, it makes sense to have them be a reflective based skin like described in the newest chapter of Silvana.
With the idea of luminescent type approaches being typically regarded, this story will be using crystallines to help determine the 'Angels' of this world.