Deltarune crack theory time, wahoo.
Ralsei is the prince alone in deepest dark, Noelle is the girl with hope crossed on her heart and Susie is the cage with human soul and parts.
Noelle being the girl with hope crossed on her heart is already treated ground. Pure crystal + thorn ring makes a sword and the girl is depicted holding a sword. Wierd route connects with LOVE finding its way to the girl. All that stuff
But I also believe Susie to be the cage with human soul and parts for the following reasons:
The mural is depicted with what seem to be claws. Cool.
The word cage does not feel very aplicable to Kris. They sure seem to be stuck with the soul but more as a vessel. Though consider what would likely happen if the soul ever atempted to control Susie. She has time after time show to be able to refuse the will of the soul, if the soul were to try and control her she would like still have a lot of autonamy. Plus, given that Kris can rip out the soul it seems that the possesed can exert influence on the coming and going of the soul.
Thus, if the soul were to try and control Susie it is likely that she would be able to both keep her autonomy and not let the soul leave. The soul would be unable to leave nor exert its will, like its stuck in a cage.
Given that Susie was able to bleed at the end of chapter 4 people have begun to speculate that she could be part human. If this is true she would also meet the "Human parts" part of the mural.
She would even fit better than Kris by a little. I'd say "with human parts" is much more descriptive of something which is part human than an actual human. That or the prophecy just struggled with comming up with a good rhyme.
It would fit thematically as a climax to Susie being able to refuse the player any choice. I feel its too big of a part of her story to not be relevant in whatever her stories climax is.
A sequence of the game where the player is controlling Susie and they are only able to take actions which she aproves of would go very, very hard. Massive aura potential. The player would control Susie but Susie would then also control the player.
So what is up with Kris then? Not too much is know about their motivation but I do believe that they could have inserted themselves into the story somehow. Perhaps they Intercepted the soul somehow, Maybe that is why they seem to be hiding the soul from Susie. Maybe? At this point it's still ambigues enough for any number of interpertations to be feasable so I'll just leave that be until next chapter. Assuming next chapter doesn't debunk this theory.
I don't fully know how this would impact the story as there are still a lot of unknowns with regard to Kris' motivations and the ending of the prophecy but I do expect Susie and the soul will interact a lot more in upcomming chapters.
Perhaps the player will need to defeat her or get her out of the picture in some other way in order to continue the wierd route. And if they don't Susie will intercept the soul trying to control Noelle and then she will force the game back on the normal route.
Maybe she can be possesed by the soul without any of the wierd route prep work cause of the half human thing?
All the same, I predict Susie is the cage or at the very least will at some point in the game trap the soul inside herself without the soul being able to control her.