In Chapter 3, Tenna talks a lot about the memories he and the family used to share watching TV (an being example when he says "you and your BROTHER," while the text "BROTHER" is white, fuzzy, and plays "His Theme". While it was unknown at the time, the TV was watching back, a lot of information is revealed. Asriel and Kris had a very close relationship, as Tenna describes. Playing and having fun for hours on end, a million emotions felt. After Kris reveals Asriel went to college, Tenna seems shocked and distraught. We know that "Ralsei" is an anagram of "Asriel", and Ralsei resembles him as well. After all, the question is "Who exactly is Ralsei?"

In the beginning of Chapter 3, Ralsei describes the Dark World as nothing but pure fantasy and illusion, claiming that Lancer is a regular card, while Queen is actually a computer. He even says he is a Darkner as well. If this is true, Ralsei has to be an object in the Light. But what?

My theory is this. Before Asriel left for college, he left a little something behind for Kris. Intentionally. He wanted Kris to remember him, knowing his next intuition wasn't going to last forever. He wanted Kris to at least pretend he's there. A reminder that he was still alive, and he'd be back someday. But, what could he be?

A costume. Ralsei is a costume that Asriel used to wear when he and Kris were younger, and they'd play. While it could be for Halloween, or just regular role-play, it is still a costume. The reason why Ralsei and Asriel look so similar is because it is what Kris remembers of his brother, but Kris has forgotten a few details, hence why there are differences between them. Ralsei losing his hat is a Dark-World parallel of Kris slowly forgetting things about Asriel, as the player has forced Kris to be their vessel.

Conclusion? Kris misses Asriel, and is slowly forgetting about him, which is why Ralsei exists.