Kong, the Legendary Marine and the Former Marine Fleet Admiral in Roger’s Era, is the big brother of Monkey D. Garp. One of the reasons that I think that this is the case is because that his name is “Kong” as in “King Kong” the great Gorilla in movies.
Monkey D. Garp Refused to be promoted to Admiral status because he will be working directly under the Fleet Admiral, and also, he would have a great chance of being the Fleet Admiral Himself which will make him serve directly under the Gorosei. Seeing that his big brother Monkey D. Kong had no power to change the Marines Even with the highest status, Garp refused to be promoted and remained as a Vice-Admiral in the marines.
The only evidences we’ve got is that name, “Kong” I still can’t be sure about it because, only a name can’t be an enough evidence to convince people that this Kong guy got some relationships with our monkey family.We have Dragon,Luffy and Garp. And their names got no relation with monkeys.But my feelings tell me that, this theory is somehow true. My mind says it got a high possibility to be true.But I just accept that “Kong” part.So even with these few evidences we’ve got, it still has a high possibility to came out true in the future.