So if the king was a prince why does he have to give the throne back? I'm confused Σ(っ...

i think because he was not a crown prince... i'm not really sure either

I'm thinking maybe he was the previous king's brother?

Because Hakuyou’s story this chapter doesn’t go too in depth this chapter, I’m thinking that he is only a part of the imperial family because his family line being the incarnation of Byakko. I think that implies that there will also be future characters that are also incarnations of other mythical beings that will also be apart of the imperial family?
That being said, him and possibly future characters would not be of direct heritage or blood of the late emperor so he isn’t considered as the crown prince or next in royal succession. Hakuyou however is the most qualified to currently succeed the throne so he is temporarily the reigning emperor until the crown prince is of age to claim his rights to the throne.

to sum it up TLDR; Hakuyou IS a prince, but he not directly related to the crown family therefore he isn’t considered to be the crown prince and doesn’t have the current rights to succeed the throne.

Hopefully this helps out a little bit? Hopefully it’s not too confusing to understand either LMAO, but I hope this helps out just a bit.

I think he is just a substitute to the real one crown prince even though he is a prince himself but according to him in he's story he's a fake emperor because the rightful one is just a 3years old kid he still not able to rule the country plus if the emperor is just a 3years old if he sit on the throne immediately many people will take an advantage to attack thinking that the emperor is a kid that is what the enemy think that way so in first to secure the country they needed someone to act as an emperor to rule the country or kingdom will not fall in to ruined! Until the real chosen one to be an emperor will mature meaning to reach in adult hood that is the only way he csn safely ascend to the throne !untill then he himself will be there to act as an emperor for the sake of the country plus he dont an supporter just because hes mother passed away early ifhes mother didnt die,i bet he will gain more supporter and he will be an emperor for real and he dont need to act as fake emperor!