What I have a problem with is, apparently Credo, Nero and Kyrie grew up in the same household; and Kyrie is Credo's sister. So to "implant" Kyrie with a demon, or turn her into one, Sanctus would have had access to Nero as well - in which case is questionable that his quarter-demonic nature would have eluded Sanctus. Granted, there is the possibility that Sanctus knew of it but didn't act because he was already hoping to lure Dante in, and Nero was under care of a loyal knight so he wasn't going anywhere anyway. In which case he may have well planted Kyrie to make sure Nero doesn't go anywhere.