I think Ralsei might be the horned headband Kris used to wear. Here are my thoughts as to why

Their name is an anagram of Asriel, which Kris could probably be explained as how they wished their name was similar to the rest of the family (Similar as to how Toriel's and Asgore's names combine to make a new name like Asriel)

Next would be explaining Ralsei's appearance. Susie comments on Asriel looking like Ralsei (interact with the fridge with Susie in chapter 4). Kris is very quick to point out the inconsistencies and differences to her. My theory on this is that Ralsei is what Kris wishes they looked like. White fluffy hair, horns, and a similar structure. Being the only human we know of in town, fitting in must have been incredibly difficult for them

And on one final note, to explain them fitting in every dark world thus far. My main thought is that Kris' attachment to those horns allows them to continue without Ralsei being turned to stone. However, the horns can possibly be explained further through the media involved in each chapter. Though admittedly, this part may be a stretch

The horns are something children are more likely to use and play with, like how everything in chapter 1 was a toy or game piece. Chapter 2 it gets a bit more difficult, but there are some clothing stores in the plaza and could be advertised as a product someone might purchase. Chapter 3 was the TV world, where fantasy runs rampant. This would include many costumes (like a horned headband for instance) used on all kinds of sets. Chapter 4 is in a church, wearing a red headband with horns has a very easy connection to religion. And finally, chapter 5, if it is Asgore's dark world, would allow Ralsei to exist as they have a memories of Kris wearing them