Rias's Pet and Protector (Demogorgon and Highschool DxD)

Long ago fallen angels learned of a place called, "The Upsidedown" and learned about a beast named the Demogorgon. A deadly predator that can teleport from the Upsidedown to the real world at will to any place. The fallen figured that if they could control this beast then they will be able to finish off the devils once and for all. They soon were able to create a portal to the Upsidedown but could not find a Demogorgon but instead found something called Demodogs. Looking like the Demogorgon they thought it could do the same thing so they captured one and tortured it to make it submit to them. But they soon figured out that the Demodog could no create a portal so they disposed of it and left it for dead. Wounded in the forest but then was soon come across by a girl with crimson red hair and blue eyes. This girl was Rias Gremory, a devil and heir to the Gremory household. She took the Demodog in and healed it and raised it. Soon it became a Demogorgon and has become her pet and protector. The Demogorgon has protected her for all his life and the only ones who know about it are her peerage(besides Issei since this is before he is a member) and her Brother and his Wife.

I do not own Highschool DxD nor the character species Demogorgon.