User blog comment:B214/Question & Answers 5/@comment-207.96.205.2-20160517060930/@comment-1359322-20160518040824

Ise's soul is that of a human not Devil, he was born a human, his soul is that of a human. It is not a soul of a Devil. The Evil Pieces is never stated to have the ability to convert souls into Devils.

Regulus is the spirit/soul of the Regulus Nemea Longinus, if the soul converts when being reincarnated, then the Longinus would be gone already. Besides if Ise's soul becomes a Devil then he wouldn't be able to use the Boosted Gear anymore, Sacred Gears can only be used by humans and half-humans. Reincarnated Devils aren't pure Devils in blood and soul.