Responses
*yuu
We don’t actually know how Seraphs work yet. The crosses have something to do with it obviously, And the Demon Army is trying to make them. However, we don’t know if the Seraph power itself was created or not, and off all current Seraphs are experiments. Yuu also seems to have been non-human before he was expiremented on (if ancient Yuu is the same as present Yuu), so we don’t know what up with him yet either.
Maybe that little you is the salt king or whatever? What if the Serapis were all adopted and died or somethings?