So I've been typing in this channel for a while now, and I think it's time to present my first ever created theory! If you are a frequent reader of this channel, you might recognize something similar I've said before, but I wanted to sum everything up in one place. With that being said, let's begin!
The possible origins of Ralsei

Disclaimer: This theory is based on Raft's theory about Darkners being some sort of toys or other objects that represented in the Light World as inanimate items. Honestly, I think this is making much more sense then Dark World being imaginary, but this is my take on it.

Ralsei is a darkner. But who are darkners? I think they are items that similar to Flowey were brought to live with determination. It seems that in Dark World, they are very similar to their original forms ( i.e. Lancer being playing card (He calls himself "Dark Jack Lancer", Seam being a plushy toy, etc ).

But what would Ralsei origin form look like? I think the answer to this question lies in the dummy's design that Ralsei made. It has
1) Hat
2) Glasses ( Made with buttons )
3) Scarf
The only thing dummy is missing is pink horns, which was probably too hard to reproduce so Ralsei decided not to include those.
That makes it 4 items in total. But Ralsei is one, right? Why would 4 items turn into 1 person? I think it's because all of those 4 items are part of a doll. It makes sense if you think about it and I can kind of prove it with this line of dialogue that Rudy says to Kris ( this happens after you inspect the angel-like doll ) :
* Oh, right, you and Azzy tried to make one too...
* But you wasted the whole time making huge wings for it!
I'm not saying that he was talking about Ralsei here, but it's important to note that means Kris and Asriel used to do stuff together, possibly including school projects.
There's even unused dialogue in the game that suggesting Ralsei at least knows what is a school project:
* Seeing you two scheme
 together...
* It's a bit like a group
 project in school, isn't it?
We know that when the game takes place, it's hard for Kris to find the right partner. I think it's because they used to work on a project with Asriel until he left for college. It also would explain why Ralsei looks more like Asriel and not Kris. Asriel
Now let's take a closer look at the parts of Ralsei individually and try to find their origins:
1) The color green, robe and a hat. This one seems to come from a drawer in Kris's house. If you inspect it you get the following message:
* There are crayons in the drawer. Their labels have long faded, and there's no green.
2) The glasses & scarf. This is the only part I can't quite find origins of. Yes, Toriel wears glasses, however, they are quite different than Ralsei's and it seems that it's impossible to trace origins of it. However, my best guess would be is that the glasses are Asriel and scarf is his too.
3) Pink Horns. This one is the most interesting and its origin is a headband that Toriel gave to Kris. Here's a direct quote:
* Remember when you were little...
* You asked when your horns were going to grow in?
* So we bought that headband with the little red horns on it.
* Oh, you wore it for months! Whatever happened to it?
However, the horns were red, so how come the Ralsei horns look pinkish?  I would say that because a long time passed, the color started to fade, and the red transformed into pink.
But why would Toriel question what happens to Kris's horns? I think it's because in her class she uses markers instead of chalk, so she has no reason to go inside the school's supply closet.
And if you are not convinced yet, here's extra proof:
When you first meet Ralsei, he says that he was waiting for the heroes a long time to arrive, and all the items I listed have tidbits of information that implies they are very old. That also makes sense, because Asriel now is in college, so the only chance of making Ralsei was when they both were little.