When issei was slaughtered by his girlfriend... Instead of Rias appearing, Lucifer appears. You see, the magic circle that Rias gave was mistaken for the one that Lucifer was given. "Interesting... But for a human to be involved with the supernatural... I can't allow for this." Lucifer, feeling empathy and deciding to do the best thing for the human, used two mutated pawns to revive Issei... There must have been a reason why this human was involved with the supernatural.