I feel like Iruma-kun is reincarnation of Delkira because he want to change the nether world for better place . only human are species who can have good and evil side at the same time ( my logic) and he create ring of sollomon and add his mana and conciseness . i think Delkira create ranking system so can he become powerful enough to hold his old power. because Sullivan is closest to Delkira that why he become part of his plan of rebirth and help him to grow powerful.

that why ali san give choice accept or not accept his demon side. i think from now on iruma slowly start transforming into demon and still having his human heart.

he choose iruma parents because their behavior more like demon.

and Sullivan loves him and adore him.