If we say that the original Axel turned himself into a nest, then the Guardian is an egg within this nest. He continuously absorbs the original Axel's mind, memories, and nutrients, nurturing a better, more perfect, crazier, and more genius form of himself.
Just like this, the original Seraph Aether True Body was merely a remnant of the original Axel, that egg, that 'Lightforged Realm' that, via brainwashing all of humanity, kept elevating its own belief, that Inside it, the divine bird being nurtured is now truly the Aether True Body of the Protector!
He killed himself, nurtured himself, and birthed himself.
This is Rebirth in Death and rebirth after death.
The new Axel is no longer the [Guardian].
But a true [Celestial Flaming Phoenix] that can illuminate the world and burn the world!
