I didn't imagine things getting hasty, but okay. I fast-forwarded in my rewatch to gather
the story of this character, from back when he turned from a human to a Flame Haze.
About that Spanish translation I mentioned early in the thread (here's their
FB page). I went to get the bit about his story, and it's the same. Here's a basic Spanish-to-English translation:
When the prince was about to be devoured, he heard a voice "I am one of those who hates those that devour men. Discard everything that you are as a human and become my vessel; in return I will give you great power to destroy the man-eaters”. The prince threw it all away, and even though he felt those words were ominous, he accepted the payment.
The prince obtained enormous power, so he used it to drive away the monster. He thought he had become a hero, he would save the King, he would save his father, he would save the country, he would drive away the monsters, but the instant he turned towards the king...
The king had forgotten everything about the prince, the soldiers too, no one in the country remembered the prince anymore. He had ceased to be a prince, ceased to be a human, and inside him a strange voice resounded "You have discarded everything as a human. You agreed with that, right? It's what you wished for". The prince had been left alone in the world.
A Denizen devouring a person's Power of Existence is basically the first half of the process of a human turning into a Flame Haze. In both cases, humans are deprived of the entirety of their Power of Existence, aka everything that makes up their humanity, history, identity and existence in our world. Flame Hazes, by extention of being linked to a Denizen by contract, can also manipulate Power of Existence in the same way. The Denizen in the contract devours the human's PoE, but instead of stopping at just erasing the human from existence, they merge with them offering them power through the energy that resides in that Denizen, aka the energy from the Crimson Realm.
Manipulating PoE includes and translates into manipulating those concepts, as those are linked. We know Flame Hazes can actually do this, not just Denizens,
with this.
Here are two extra moments around
Shana becoming Yukari Hirai. Everyone else calls Shana "Yukari" or "Hirai", even the human characters who later learn about the truth of the world, as their discovery happened after the real Yukari's existence erasure. Also, despite Yukari and Shana's height difference, with Shana being identified as "Yukari" by the entire world, Shana's height became the "normal height" of "Yukari", same with their personality. In the prequel OVAs, Shana had also become someone else by merging with the remains of their existence, after that person was erased.