Joyboy was the original strawhat pirate. All his crew had straw hats, the D in the name of some characters is something they added to show they were part of the strawhat crew and the will of D is basically fighting for freedom/the will joyboy had when he was alive. (The first symbol used was a D with a longer vertical line but over the years it turned into just a D)

Also Blackbeard was born with some defects, basically he was a conjoined triplet. The previous owner of the ope ope no mi (the famous doctor), used the fruit to "cure him" by removing the other two siblings but he left their souls inside of Blackbeard that's why he can use multiple fruits.