Based on what Shanks and the Chief of Fusha village say when they reappear in Chapter 96, Luffy was destined to become a pirate. Shanks celebrates the occasion for reasons yet unknown.
In Chapter 154, Dr. Kureha corrects a Drum Island villager when he says the name “Gold Roger”. Being well over 100 years old, Dr. Kureha was alive at the time of Gold Roger’s execution. Kureha tells the villager that his REAL name was Gol D. Roger. Over time, people shortened his name to Gold Roger and forgot his real name. It is no coincidence that Luffy’s full name is Luffy D. Monkey.
Luffy has a 'brother'. His name is Ace D. Portgas . Although his identity was not revealed, a Drum Island villager first mentioned Ace in Chapter 154. Just a short while later, Ace first appeared in Chapter 157. He is a member of White Beard’s Pirate Crew, perhaps the strongest crew in the world. Actually, the two are only brothers in name only. They are not related by blood.
There is another man who is somehow related to Luffy. He is the leader of the Blackbeard Pirate Crew and his name is Teach D. Marshall. Obviously he shares the same ominous “D”. Luffy first met Teach at Mock Town in Chapter 223 but his identity was not revealed until Chapter 234.
There is yet another character we know of who holds the "D". His name is Saul D. Hagwar, and he's a giant! Hagwar's giant race has not been specified as of this writing (Vol.48) but he is not a member of the Elbaf breed of giants.
And believe it or not there's another character who.....you know what....this is getting absurd. Okay, if you have not read Volume 45, do yourself a huge favor and just stop reading now. I mean it. There's a surprise waiting for you that's big enough to blow your socks off and through the wall if you're a beginning reader.....
You're not gonna stop are you, okie dokie, don't say I didn't warn you. There are two more characters we know of that share the "D" and they are directly related to Luffy. One is Dragon D. Monkey, Luffy's father and the other is Garp D. Monkey, Luffy's grandfather. Both are characters that have shown up before in the series prior to being revealed as Luffy's relatives during the very exciting Chapters 431 & 432. Garp is the man responsible for training Luffy and Ace, molding them into the beasts they are today. Some of Luffy's training included: being tossed off cliffs, thrown into jungles in the middle of the night and tied to a balloon left to float away. Luffy didn't know that Dragon was his father until Garp told him upon their meeting in the comic and wasn't even aware of the infamy surrounding his father's name.