METAMORPHOSIS.

For starters, most people that try to figure out what ralsei is in the real world are basing it off of this appearance

however, I feel like there's plenty of evidence to point to this not being his real form, right? People have already pointed out that his original shadowed form isn't fully consistent. It's possibly the most obvious when you compare his singing animations in both forms. His hat form makes what was later 'revealed' to be his ears look more like hair?,

Ears don't really split the same way that hair does, and theres other examples of hatsei having this kind of spikyness to his 'ears' that hatless ralsei doesnt have.

even the fangamer plush makes his ears spiky!!

its a pretty major part of how hatsei looks, and its certainly been talked about before. And then comes chapter 3+4. And we have plenty of evidence that ralsei is a shapeshifter, and I have seen literally nobody talk about it????? huh?????

Oh, and the hat casting a shadow on him makes no fucking sense because he goes onto wear SEVERAL hats in chapter 3 and he's normal????

also I know its like. A funny bit, but HE TURNS INTO A HORSE

WHY THE FUCK WOULD KRIS'S HEADBAND TURN INTO A HORSE???? WHY WOULD A GREEN CRAYON TURN INTO A HORSE???? WHY CAN HE DO THIS????? THIS ISNT A COSTUME THATS NOT HOW THEY WORK????? WHERE WOULD HIS BODY GO.

not to mention that changing shapes was literally his ability in the legend of tenna game???? he plays it off like 'oh every character has abilities i can turn into a box' but he can also turn into a dog? since ralsei was the only one who read the manual it very well could be an ability given to him since the real Ralsei is also a shapeshifter.

It would also explain why ralsei draws himself in his hat form

thats closer to what his natural form is. Dont have any screenshots on hand right now, but he's got two lines in chapter four (if you leave him lying on the ground for too long, and right before they find the first fountain) about how much longer he can 'keep this body for' that make it very obvious that he's only using a form that looks cuter to appeal to us. Him being a shapeshifter would also explain things like

His face being a deliberately made abstraction would also make this interaction make a lot more sense. Pre chapter three, I assumed Ralsei based his face on Asriel to either try appealing to Kris or as fanservice for the player/red soul, however, now that we've slowly started learning more about Ralsei, it's beginning to seem more like Ralsei just wants to have a face and more distinct appearance, like the lightners do. However, because of how dark worlds work, he can only base it off of what already exists, with that already existing 'model' being Asriel, although with modifications to make himself cuter— pink horns and eyes, and his usual glasses. It's why Kris is always quick to point out differences between them, and why Ralsei is embarrassed at being told that they look similar, he didn't have a choice other than be based off something that already exists.

Alright, so Ralsei is a shapeshifter. He still has to have some equivalent in the Light World though, since that's how Dark Worlds work. He was literally about to tell Susie what he was before getting interrupted, and Toby Fox is deliberately dancing around the topic.

However, I think the answer is actually pretty obvious. Ralsei is a being of 'pure darkness', which is why he can exist in any Dark World, unlike Lancer and Rouxls, who need to be objects that 'belong' in their respective worlds. His form is made up by the original dark fountain, and he describes himself as a 'Prince of the Dark'. Characters in the Dark World know about what happens to and around their real world equivalents, but Ralsei in particular seems to be especially aware of all of Susie and Kris's actions and movements. He doesn't need to be brought in by Kris like Lancer and Rouxls do, and he always appears in the Dark World a few moments after Susie and Kris do, while somehow almost always having pretty intimate knowledge of how the world came to be. Ralsei is also the most adamant on being depended on by Lightners, even more than people like Tenna. He talks about how a Darkners role is to be used by Lightners and to make them happy, and his character development in Chapter 3 especially goes into how he wants to be needed and how he's afraid he's slowly developing his own personality, and why he believes darkners shouldn't do that.

So, taking all of that into account, I feel like the most obvious answer for what Ralsei is is a shadow.

He's a literal prince of the dark. It explains why he can shapeshift, since shadows can be made to look like anything— I'm specifically thinking of things like shadow puppets, and why when he gets knocked out he seems to literally disappear, returning to the shadows. A shadow is also the most dependant on light, shadows literally cannot exist without light, or they'll just be darkness. It even explains his empty room.

His insistence that his only role is to help the Lightners, the way that people can never find anything notable about him (asking swatch for specials his suggestion for Ralsei is based purely on how he dresses and Queen literally forgets to get him a cage), and his ability to be in any dark world (since there's literally nowhere without shadows) all seem to point towards Ralsei being a shadow.