Er.... There's a 'theory' that the body effects the mind? That's just known though. Hormones (esp pre, during & post puberty), synaptic pathway formations, brain damage.
That said, in this manga, the soul does appear in fact to have it's own set of memory & mental faculties, given he's met the gods several times in a soul body and could think & talk & all that, even when he was dead. So there's however that interplay works as well.