>Ralsei is an anagram for Israel
>his color scheme matches the Palestine flag
I think that Kris and Asriel had a falling out over their differing opinions on Israel. This escalated to violence, leading to Kris killing Asriel and hiding his ashes.
Ralsei is a Darkner made of Asriel's ashes, but an incomplete set of ashes, so he has a different personality.
I think that Asriel was pro-Israel, hence the name. And Ralsei is Kris's imaginary version of Asriel who would have never gotten into a disagreement in the first place, and thus supports Palestine, hence the colors.