http://www.astraeasweb.net/plural/glossary.html#soulbond For a source that doesn't consider the idea mock-worthy. It's a fairly obscure term that comes from the Plurality / Multiple community. Once you've accepted the idea that you can have multiple personalities inside your head, this extends it to the idea of having some fictional character from another source as one of those personalities.

(The otherkin link is Wikipedia, which seems neutral enough as sources go :))