Anon says:

Add is Add. There could be multiple branching timelines, but they all must meet at an origin point somewhere before they split off. Considering Eun has memories of helping Add in the past, it seems illogical to conclude that it’s a different Add this time than it was before. Indeed, Add’s own subconscious memories such as knowing Ran’s name in Elder and etc imply that indeed it was the same Add who failed all those times.

Diabolic Esper being D has been theorized by many for a while now, and in my opinion, this does not seem like the direction the writers wish to take. It seems a bit too obvious. Indeed, they easily could have made a few references that seem to imply DE as a red herring to keep the true identity a surprise.

My guess is that D is one of two things:
1. The final boss of the game. The boss they decide to implement at the very end of PvE when they have their mind made up that new dungeons are done. This could be the Secret Dungeon boss of Elysion.
2. A completely original character specific to this light novel series.

The first option would mean that the game story could tie into the light novel, as the novel series could function as a sort of sequel where D struggles to get revenge after failing the first time. This is backed up by Aisha’s revelation of having achieved her great power through great sacrifice.
The second option would be interesting as well, as an original character entirely would allow the series to hold up better on it’s own as something worth reading if you are a fan of the game.