There's not a lot we don't know about One Piece's protagonist Monkey D. Luffy. However, for the longest time, fans didn't know who his father was. Many speculated that he was the son of Gol D. Roger, the previous King of the Pirates, but as that was disproven, his parentage remained a mystery. Luffy was raised by his grandfather Monkey D. Garp alongside Portgas D. Ace and Sabo, and his father was never really brought up. But eventually, we finally got to meet the man.
Monkey D. Dragon is the son of Garp and the father of Luffy. He is known as the "World's Worst Criminal" and he is the Supreme Commander of the Revolutionary Army. His goal is to overthrow the current World Government in order to undo the oppression that it imposes on the common citizen. As the army's leader, he has the support of many countries that oppose the systematic oppression of the World Government. He holds the respect of his entire army, despite his secretiveness about his family and past.
While Dragon was never truly a part of Luffy's life, he still clearly cares for his son. Ivankov notes that Dragon constantly looks towards the East Blue, as if someone he loves lives there. Despite his appearance and stern nature, he cares about those close to him. He is supportive of Luffy's status as a pirate and his dream of becoming the Pirate King. When it was revealed to everyone that he is the father of Luffy, his comrades did everything they could to protect Luffy when he attempted an escape from Impel Down.
It's not known why exactly he left Luffy in the care of Garp, but he would have no contact with either one until seventeen years later, when he encounters Luffy. It could be that his father's connection to the Marines drove Dragon away, but why he left Luffy with him is a mystery. While Garp is loyal to Dragon and Luffy, he is equally as loyal to the Marines. He doesn't believe that bloodlines determine one's fate, and that being a criminal doesn't make someone a bad person. This was proven when he left his grandsons with a mountain bandit named Curly Dadan. It could be that this, along with the fact that Garp also despises the nobles of the World Government, is why Dragon entrusted his father to take care of Luffy.
While Dragon was an absent father to the point that Luffy neither cared nor wondered about him, he is shown to love his family, including his father Garp. While his reasons for leaving are a mystery, it doesn't seem to matter as his actions in the present seem to be more important. Luffy has accepted that Dragon is his father, and doesn't dwell on what could have been, so that's that. Dragon supports his son's dreams and actions, and he wants to make the world a better place for everyone. So while he wasn't a great parent, that doesn't mean he's a bad person.
