Unlike Akane's shinigami, who's a separate being, Kaisei's shinigami is actually himself.

All his suffering is contained in that vessel. 

The reason we see it in the form of his adult self is because Kaisei was forced to grow up much faster than most kids. Not just because of his family situation, but also because of the weight of his name. He has to be better than everyone else. So he feels like he has to grow up and improve before anyone else does.

Having to watch his mother suffer because of their debts. Knowing his family is poor. Knowing his father abandoned them to start a new family, and that the rest of their relatives abandoned him and his mother as well.  Being just a little kid, unable to do anything but smile at a mother who can't stop crying because she feels she can't give him a decent life.

Throughout the entire performance, Kaisei has been talking to his inner child, that innocence we all carry somewhere deep inside. That's why the protagonist is portrayed as a child. That child is the light Kaisei still has left, the key that will allow him to be reborn and, this time, live for himself. Not to prove to Issho that taking him on as an apprentice wasn't a waste of time, nor to carry the weight of what his name means. 

That's why I love this scene so much, the one where Kaisei talks to his mother. It's here that he realizes he doesn't have to let his name define the way he lives, that he doesn't need to be the best or the first flower to bloom. And that's what I love most about it, because at this moment, the story gives his name a new meaning. It no longer represents the obligation to be the first; it represents the chance to be reborn, to bloom into a new person.  Until that moment, Kaisei hadn't truly been living; he was nothing more than a shell built from his own suffering. It wasn't until he finally let go of everything that had been weighing on his heart that he truly began to live.

I really love the symbolism of the candle. Taking it would mean going back to where he came from, going back to living for other people. But this time, he has to make his way through the dark cave on his own.

As a side note, in the official Spanish translation, when the Shinigami says he no longer needs the candle, he actually says, "It seems we don't need it anymore," referring to both of them.

Another aspect of this performance that I love is how it makes use of the manga's black andwhite art. At first, the shinigami is depicted in a black kimono, reinforcing its connection to death and suffering. But the moment Kaisei embraces himself, that very same figure is transformed, taking on an almost angelic appearance as though it were now clothed in white.