With us being 4 Chapters in, halfway through the game, and still not knowing what object Ralsei embodies, I think part of it could be due to his Light World form being something unusual, but I think another, possibly bigger, reason for this narrative choice not to reveal what he embodies has to do with the theme of Darkners being people, despite what he tries to tell us.
Ralsei claims that Darkners are just objects, that they don't matter, that they love serving their purpose, and in Chapter 1 he even said that being useful to Lightners is the only way for them to be fulfilled, but Susie has consistently questioned this or shut it down; this is one way that we as the audience are nudged towards questioning it. Darkners are also written with the same complexity as Lightners, and most of them are content in Castle Town, where they're just living their lives in the Dark World and forming social bonds with each other, largely away from Lightners, and not being "useful".
Even Tenna, another lonely Darkner with low self worth, who reduces himself to just being a TV and claims he was happy just serving his purpose, indicates that he has dreams beyond it. As the Chapter 5 cutscene shows, his unhappiness in Castle Town doesn't come from being useless, but rather from being lonely in the Dark World; he lost his partner (the only person that truly understood him), and then seemingly lost a friend as a result, being left all alone. He dreamed of having a family with Spamton, he dreamed of traveling... he has desires beyond his purpose, and when his survival doesn't depend on being useful, those desires actually appear to matter more to him.
We're clearly meant to question the idea that Darkners are just objects, that they have no desires outside of their purpose, or that serving their purpose will give them a truly happy and fulfilling life. Especially when you consider the existence of a Darkner like Spamton, for whom being useful to Lightners is outright impossible; he's spam mail, doomed to be ignored and useless.
Ralsei himself wishes he could join Kris and Susie on their adventures in the Light World, wishes he could do their homework, and is starting to develop interests and opinions that aren't in service of Lightners; the previous lack of those wasn't due to him naturally being just an object with no real feelings, but due to him repressing himself because he believed he's not "supposed" to have any real personality or desires. He also claims that being useful is what makes Darkners happy, but we see that the things that actually make him happy are spending time with his friends and being treated like a friend rather than a servant; being comforted when he's upset, being told it's okay not to smile... he's not being "useful" there, and his Lightner friends are actually caring for him, but it makes him feel better.
He's clearly not just an object. He's a person. And I think the choice to not reveal what object he embodies is, at least in part, meant to emphasize this, along with his visual design.
If the player doesn't know what object Ralsei embodies, that makes it harder for them to reduce him to just being said object. There are theories about what his Light World form is, but we still don't have anything concrete. The player only gets to knowingly interact with him in the Dark World, where he is a person. His Dark World form strongly resembles the Dreemurrs, which is brought up even within the narrative itself, where he's especially noted to resemble Asriel (their names are even anagrams of each other). He could easily be another child in the family, and he wouldn't look out of place; the pink horns are a small enough difference that they could be assumed to be some kind of fantasy condition or just a color variation.
In the Dark World, he looks like a lost Dreemurr kid, and he acts like a kid. This, along with our lack of knowledge of his Light World form, makes it harder to believe him when he says he's nothing more than an object, and makes it more apparent that he's an unreliable narrator.
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