Henry.
The killer uses letters to scare his victims, Henry wrote a very similar one to test Trish.
Henry didn't take at shot a Wakefield, although he was shown as being a great marksman earlier.
When Trish was almost drowned, he magically appeared just after Richard saved her.
Henry was the one, who decided to marry on Harper's Island
The only person that Wakefield didn't attack in the show right now, is Henry.
Henry was there, when JD was killed and he was covered in blood...and it was never explained. He just said he found him. Yet he would leave his brother to bleed to death there? Only to return when Abby finds him?
He has a violent streak. He attacked Shane. He tried to lure Trish to Hunter's room in the hotel, and we don't know what would have happened if she went inside. He hid in Hunter's closet with a freaking knife! He was the only one who knew Hunter was leaving other than Thomas. And the killer left a shotgun trap on the boat Hunter used.
We know he was there, on the island before, as a worker on the Wellington's boat.
Now watch me, being completely wrong July 9!

But after last episode, they pratically said it was Jimmy. He's obviously just a red herring. Or it was the big reveal and I just didn't realize it.