In Chapter 3, Ralsei explains how the dark world works. She mentions all darkners just being objects in the real world. There is also a popular theory that Ralsei is Asriel, just cuz of the similar appearance, and also the name being an anagram.

I thought, whenever we see kris in the real world, they're wearing a shirt similar to Asriel's shirt / jumper (Green and yellow).

I'm thinking that the shirt / jumper is the object that makes up Ralsei. It connects to Asriel, and Kris carrys / wears it everywhere in the game. The only time they don't have it is in the dark world, probably because Ralsei is currently in this world.

Only problems (and I feel like their really small problems) is that the green colours don't perfectly match, and the number of stripes on the shirt is different. I don't think this matters tho, cuz it's obvious they're supposed to look similar.

This theory is simple, and I wouldn't be surprised if someone else said the same thing. It could also be a stretch as well.