Question about soul from the anime soul eater?

He's inside his conscious/mind/soul , the little devil demon is the essence of the black blood and he's trying to corrupt Soul.

I haven't seen Soul Eater in a while... but one Anime that I highly have to recommend, is Ao no Exorcist. Summary: This world consists of two dimensions joined as one, like a mirror. The first is the world in which the humans live, Assiah. The other is the world of demons, Gehenna. Ordinarily, travel between the two, and indeed any kind of contact between the two, is impossible. However the demons can pass over into this world by possessing anything that exist within it. Satan the god of demons, but there's one thing that he doesn't have, and that's a substance in the human world that is powerful enough to contain him!! For that purpose he created Rin, his son from a human woman, but will his son agree to his plans? Or will he become something else...? An exorcist? The main guy, Rin Okumura, always use to remind me of Soul from Soul Eater. They're both really funny, and I think Ao no Exorcist is an amazing Anime! If you do give it a try, I wish you happy watching!

He's basically in his own mind. The little demon dude is the madness.