Shadows, Light, and Blood

What if Issei wasn't just a human? What if witnessing a skirmish between Devils and the Church causes a change in Issei for the better? What is this change? What is Issei? Who's this Vampire that calls him Master? I don't own Highschool DxD in anime, manga, or light novel form, I only own characters created by me. Smart, Less perverted, Strong, Reincarnated Issei.