My theory on Ralsei (and Dark World's as a whole), since I figure I may as well throw my hat in the ring.

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Darkner's can only be made by the things around you. Everyone's outfits are made of their clothing, all the residents and environment are made of the items and objects within the room, etc etc.

So what is Ralsei made of?

Well, given Kris's home life and their duel nature as both Knight and Lightner, due to being both the Heart (ie you) and themself... your Determination becomes their armour and sword, and Kris's hand-me down clothes (and the dust from around the room, ever present and soft and definitely not dangerous except when it suddenly is) become Ralsei- a younger version of Asriel that allows Kris to be the older, cooler sibling.

Some evidence and explanation for this;

- Darkners exist, per Ralsei, only to make you feel better. Kris has identity issues and feels a weird distorted bond with their brother. Friendless, always the little baby, and not a monster so must be protected. In the Dark World, they become a hero, and Ralsei exists for them, basically. He's just so nice he extends his help to others as well.

- Taking the taste tea into perspective; Noelle, who does not know who Ralsei is, but DOES know Asriel... sees Ralsei's cup as just empty. There's nothing in there. He exists purely for Kris, made of Kris's worn clothing and dust and feelings about Asriel.

- When Ralsei is KO'd, in either game, he just poofs into clothing, as if his body has just poofed away like dust. He's also very fluffy and cute, like a dust bunny (or goat, if you will)

- Again, Darkners can only exist in a world made from the room tthey exist in... or Ralsei's. Ralsei is the one exception, able to live everywhere. This suggests that something about what he is made of is different or intrinsic to darkness... like dust, which is Everywhere and Unavoidable.

- Every dark world so far has taken on aspects of the people involved and focused itself towards helping them. The first world featured an abusive **** heel dad who gets to be beaten up. Susie, we know from stuff in Chapter 2, has a pretty bad home life. We also know Kris has issues with their dad as well given... everything, really. The second world was far more focused on Noelle (and to a lesser degree Berdly, because his trauma while important isn't actually that Big compared to Noelle's) and was about an overbearing but unattentive mother who wants What Is Best and will disregard your own feelings... which is exactly how her hard ass mayor mom sounds like in real life. The "hub" that Ralsei is from is clearly meant to be a ideal town for Kris (and friends) and is more closely linked to Kris's desires. Note how Kris's room is only their own bed, but they get all the trophies and stuff.

obviously, with only three dark worlds and two chapters to go on this is all subjective reasoning. Once Chapter 3 is out we'll see if there is more support.