Okay this idea has two versions

The first u are like tapion in dokkan where u and hirudegarn work together and fight the devils who oppose u

The second version is u are hirudegarn and transform from human for that hulking beast when angered u are stronger as Great red in dxd meaning u are in par with ophis but u have awakened from your slumber after the great war after being sealed away u awaken with hatred for the Devils

starting out as your first form when u emerge then evolving into your final making your power just a bit more than ophis but that's after u encounter rias gremory

That and u are known as

THE DEMON GOD HIRUDEGARN!!!

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So that's 2 ideas in one tell me which u think would be best and which u might want a story of ^^