Monkey D. Dragon is the Leader of the Revolutionary Army, responsible for many taking down of Goverments around the world. He is crowned, with Gol D. Roger, the most heinous criminal in the world. Monkey D. Dragon was theorized to be born a Celestial Dragon, seeing what the World Nobles do to their slaves. He was brought up in the Goa Kingdom, like Luffy, Sabo, Ace. He is the son of Monkey D. Garp. Unlike Luffy and Ace, who went on to become pirates, he followed his dad's orders to become a Marine, spending a few years under the rule of his father Garp on his ship.
He was probably at God Valley at the time the Celestial Dragons began their event their (to kill as many people as possible and get 6 very insane df fruits, the strongest on the seas), Dragon probably saw the event, and could of possibly stole a fruit from the host (probably how he got his powers), and I believe that his Devil Fruit is the Hito Hito no Mi, Model: Indra after the god of the weather in Buddhism, Indra. We dont know what this DF looks like, so.. and also its just a theory.
This was probably also the reason Rocks D. Xebec went to war on God Valley, to steal the very much overpowered DF (probably those 5 devil fruits are now the yokai of the gorosei), but why would Gol D. Roger intervene in this? I really have no clue about that. Even so, he was there, he fought alongside Garp and Dragon, and after defeating Rocks D. Xebec, Garp was crowned Hero of the Marines.
He knew what the Celestial Dragons were, and still are up to, enslaving people, so probably after the war, he left the Marines and formed the Revolutionary Army, recruiting many people such as Emporio Ivankov, Bartholomew Kuma, and many more people. He was probably taught Haki by his father, Garp.
Now, with who did Dragon have a child with to birth Luffy?