(Note: This will be tagged as 'hearttypes', because that term and 'synpath' are functionally synonyms, as admitted by the coiner of 'synpath' nerself.)
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I don't quite recall who, but I remember once seeing 'synpaths' being described as 'kin for fun, but for people who are already kin'. Which I suppose is true. However, for me, and my personal experience, that doesn't feel quite right. In KFF circles, 'kin' seems to be a synonym for the word 'stan'. And my synpaths definitely aren't the characters that I stan. I've also seen synpaths be referred to as one's personal set of 'blorbos', but that doesn't feel quite right either. Again, this implies that one views their synpaths through a very fannish lens, even if it does feel as if it implies more permanence. (At least, to me, it does.)
The coiner of the term describes it as follows: "Synpath is a tidy name for something you identify with on several levels, which could be a concept that resonates really strongly with you, an animal or mythological creature you feel you act like, or a person or character you share a lot of common behaviors with, among other things." This description has always felt more personal than KFF's "kins", or fandomgoer's blorbos. To me, my synpaths are things which I identify with, as well as things that are very important to me, how I view myself, and/or my life. Because of this, I've always felt that my synpaths have always been more telling to me, as a person, than my kintypes. My kintypes are not something I choose, after all. They're my past lives; they hold no bearings on who I am now. My synpaths, though, are entirely based off of who I am today, my experiences, and my interests.