What is Ralsei in the Light World?

ralsei is actually the cell phone that kris always has. think about it. asriel is in college, kris could call asriel,ralsei is an anagram of asriel,and the cell phone is always on kris's person.

I honestly agree with this. You can see that ralsei has horns, except that they're red

Ralsei is the piece of chalk Susie ate at the beginning of the game.

Ralsei is your best friend A S R I E L D R E E M U R R

I think Ralsei is the manifestation of what Kris wanted to be. A brother to Asriel. This fits well with the horn headband since Kris got it because they kept asking when their horns would come in.

I have another theory. Since ralsei's kingdom is a place where people from both other dark worlds can visit, and ralsei can visit other dark worlds, my guess is that he is somehow literally just from the dark world, with no real counterpart, and that his kingdom is somehow manifested by itself, rather than by real objects. I mean, Lancer turned into stone in the Cyber world, so why shouldn't the people from Cyber world turn to stone in Ralsei's kingdom?

Ralsei is the Heart Locket. We see that Kris has the knife, that implies Kris has the locket. The locket contains a promise: "Best Friends Forever", the real life version of the locket has a music box, and Ralsei sings. Ralsei has a heart shaped hole in the middle of his cloak, like the inside of the locket. The real life version of the locket has the Delta Rune on it, and Ralsei is obsessed with the Delta Rune prophecy, knowing it has a meaning other than the angel's power. Ralsei's scarf also mirrors the chain of the necklace that holds the locket. Surviving in all the dark worlds fits with Ralsei being something 'native' to Kris, if the Dark Worlds are all created by Kris. Ralsei represents the bond Kris had with Asriel, as embodied by the monster Kris wanted to be.

He is one of those little green tree air fresheners you find naturally spawning in vehicles across the world.

I don't think he has a physical counterpart, I think he's just made of pure darkness like his fountain, which would explain why he can traverse other dark worlds and why he disappears when downed in battle. As for his Dreemurr like form I think he intentionally took on that form because he knew it would mean something to Kris

Whatever Ralsei is, he is almost certainly not just some random object. In chapter 2 he manages to get over to the library on his own, so whatever his lightworld form is, it can still move.

Some theories suggest that Ralsei is who Kris wants to be. Like Kris wants to be a monster so he can fit in with the others because in the acid tunnel, Ralsei mentions how Susie is mean but likes her for being herself, and also likes Kris for being themselves. When Kris tells Ralsei that they like Ralsei being himself. Ralsei tells Kris that no one has ever told him that, and tells Kris that he doesn't know what being "Ralsei-like" is like, meaning that Ralsei's behavior is not original, but rather someone else's.

Ralsei is Kris’s hidden fursuit from their furry phase

Not all darkners are physical objects you can pick up though -- several darkners in chapter 2 are computer data: task manager, paint programs (swatch), popups and ad scripts. They exist in reality (as info encoded electrically or magnetically) but they're not really objects like playing cards, per se. My theory is that Ralsei is in Kris' head. Possibly in memories about Asriel. It makes sense of how Ralsei just turns up in chapter 2, and Ralsei seems to know a hell of a lot about Kris specifically (more than he does about, say, Susie)

One interesting theory I’ve heard is that Ralsei is an idea rather than an object, being possibly a product of Kris’s desires to see Asriel again, among other things. I mean, if Darkners can be made from nothing but data (like Spamton and many other Cyber World Darkners are implied to be) Ralsei being made from an idea/wish isn’t too far fetched, and would also explain things like how he’s compatible with other Dark Worlds or how he can appear in the Cyber World suddenly.

Since he's so fluffy and Kris gets a wardrobe change in the dark world, I like to believe that he's Kris's sweater :)

Imagine after all this Toby just makes it a pair of glasses

i think its definitely the headband. in the game its explained darkners turn to stone when they dont "fit into" the dark world they are brought to. the headband is something kris wore all the time, which means they wore it EVERYWHERE. thus, ralsei would fit in anywhere, as the headband was worn constantly everywhere.

hes Kris’ sweater. hes fluffy, like a sweater. hes comforting, like a sweater. his cloak is green, like Kris’ sweater. and Asriel probably had a similar sweater when he was a kid, which would be why Ralsei is so similar to him.

I think Ralsei is a goat simulator gift card(possibly a gift from Asriel) Kris forgot inside his pocket

Plot Twist, Ralsei is secretly a Lightner.