Neji already said she was a part of her soul earlier. Honestly in Karin's position my guess would be 'dunno, reincarnation or something?'I think the big deal here is that where Karin is concerned its just a talking sword. Swords are things you find and pick up, not generally what you are born with.
She most likely thinks she ran into this magic ghost sword lying around after the owner died
[X] As far as I know, before the Hollow attack I was a pretty normal girl from a spiritually sensitive family, but I guess maybe there's more to it. Nejibana said something about how she's my 'birthright', and both she and Hat n Clogs have acted like I should already know a thing or two about shinigami, but she's not answering me right now. I don't know what else to tell you.
-[X] Suggest that you'd be happy to try and get more information from Nejibana and Urahara. When this conversation is over, let Goat Face know about it, including your plans to try and get answers - this whole day has left you feeling pretty down and worn out, and dads are supposed to be good at helping you feel better about this kind of stuff, right?