when Minato used the Dead Demon Consuming Seal and sealed the kyubbi inside his son Naruto,he sealed inside of naruto some chakra from hims and chakra from the mother of naruto,so if the seal is going to be ''unsealed'' by the kyubbi or the hate of naruto,minato or kushina will appear there,inside narutos mind,and help him surpass the hate of kyubbi an ''reapair'' the seal a little...kushina is not a ghost just the chakra of her exists inside narutos seal to help him from unseal the kyubbi..if you remember something ''same'' happened when in naruto series,and not in shippuuden,in naruto episode 183 when naruto was in mission in the hidden star village and there natsuhi,that was a woman tht complete the training of the meteor that they had in this village,make the Mysterious Peacock Method: Apparitionedit( that ''seal'' her chakra in the ground,this time, before she dies,and so after her death is able to have a body from chakra,and this is how she help naruto to win(
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''same'' happened with the mother of naruto too