Monkey D. Dragon. A mysterious man with a past that we have yet to uncover. He is the son of Monkey D. Garp and the Father of Monkey D. Luffy. His lover/wife/girlfriend (Luffy's mother) has yet to be uncovered, as is his Mother. He is the leader of the revolutionary army and is known as the world's most dangerous criminal as he intends to over throw the World Govt. He has a Cinta for freedom and he believes in fate, both of which are strong traits of a "D.". He appears to be a very intelligent man (a trait which was Lost on both his father and his son). He attended the Pirate King's execution and (though he is an absent-y Father) he seemed actually happy for his son becoming a pirate and living his life the way he wanted to. It is clear that he was never a good Father to Luffy, as the boy has no recollection of ever even having a Father at all. Any other information about the man is yet to be revealed. As I have already said, he is very mysterious. I hope to see lebih of him.
He is exactly what anda just berkata - Luffy's dad and leader of revolutionary army. I'm not sure what kind of answer anda expected, but when it is revaled, anda will find out - because Eiichiro Oda will definitely not let peminat-peminat down!