That is a good question and the answer would probably be different depending on who you ask. For me, it’s exactly what it says on the tin: a character that brings you comfort.
Most of mine are longtime favourites or canonmates. And of course, my favourite Pokemon, the unsung heroes of us all.
I don’t see CCs too often outside of fictionkind circles, and I wonder why that is, because in most fandom circles they’re pretty important as an unnamed concept. (Looking at you, Supernatural, with your Sam girls and Dean girls.)
Hope that helps? A bit?