You might also notice that I purposedly highlight Ajax's odd eye under the black veil whenever I have the chance.
So by matching the cover with the ending scenes of the story, you can say that Ajax represents The Moon, an illusion, solidarity being with insecurities.
You might've noticed that in the first part of Vol 1, Ajax is usually drawn from the left to show his odd eye. But it gradually turned to the right towards the end of the story.
When the story completely shifts to Tartaglia at the end, the right eye (Ajax) is put away.
The other Diluc in my Delusion Puppets AU is the harbinger that lives inside the Delusion Diluc inherited from his father.
The Harbinger needs a host for him to use his power and Crepus wasn't compatible :p