From the blog post,

"when you kill and absorb six perfectly innocent humans' souls in order to defeat the last fallen human the very second they fall, before they can gank anyone,

i couldn't think of any super cool boss battle form despite trying to for a while so the rat just becomes un-ratty and splat becomes visible and also corporeal so they can assist in the ass-kickening. why not"

Chara and Sans belong to Toby Fox, etc, etc.