Who is Imu-sama in One Piece?

As we know the at the center of the world the throne is where this guy called imu-sama sits the throne represents that all the royals in different countries are equal and no one desire's dictatorship the 20 weapons in the front of the throne represents the pledge that was made 800 years ago by the first twenty that created the first world system and the symbol on throne represents the 5 elders.

Five Elders Kneeling to Imu-sama

So who is this guy imu-sama what we know that he is the head of marines and world government
he is present in Pangaea castle in Mary Gorosei very less number of people know of his existence
as we saw in episode 886 it was represented as Empty Throne.So he tells about the next wanted criminal which is to be killed to 5 elders and elders tell it to the world government

Imu-sama 

As far as Assumptions and theory goes about this guy is that : He is not a human and not from this world he might be immortal .He was a living on the another planet and due to some reason the planet was on the brink of destruction so he came to earth. And when he comes to earth he brought the devil fruits along with him . So he might possess some kind of power in order create or destroy the devil fruits so it may not work on him.

Will of D

So when he came to the earth there was AK(Ancient Kingdom) which was under the clan D.This clan D was feared and admired all around the world because of it's power and overall governance .So this clan D has made a world which is free and peaceful which might be referring to the will of D.So in order to safeguard the AK they have weapons of mass destruction which might be used when anyone tries to threaten them.

So imu-sama used this opportunity to manipulate the rest of the world leaders telling to about clan D is dangerous. As they this powerful weapons.So using this leaders he defeated the clan D by declaring the war against AK. which represents the ideology of world government "killing the few by saving the many".

Poneglyphs

So AK created poneglyphs so that there successor will decipher it and take there revenge on world government and as we know that some poneglyphs tell about  void century other tell Red ones tell about tell about Raftel

Devil Fruits

While the war with AK imu suffered loss of devil fruits which further got scattered around the world.
So about the big straw hat it might be the hat used by leader of clan D. At that time.So as the hat is now with luffy so he and his crew might be the one who will avenge the clan D.