I though of Don Creek at first, later on Arlong, and now maybe Morgan.
Those 3 are a possibility.
Anyway... none of them are a danger for the Strawhats, they became too strong.
Luffy would take all three of them out at once right now. Just once Gear 2 and done.
But maybe he has a DF ability now... who knows.
^ it can't be him IMHO. This guy is a righteous marine to the bone, he just can't have fallen this low just because Zoro beat him and his men !
I am sure we will see him again in the future though.. He would make a great addition to Smoker's men.
The person is looking for the Straw Hats so I don't think it's Morgan because he only battled Luffy and Zorro. There wasn't even a real crew at that point. hough it wouldn't surprise me if it were.
I think Arlong is done and I doubt he would use a crew of regular humans.
I'm thinking it was Creek. He was defeated by the core members of the Straw Hats (Luffy, Zorro, Sanji, Ussop & Nami) and he always aspired to return to the Grand Line. He would hate the Straw Hats because of the way he was defeated and the fact that they stopped him from getting the ship he wanted in order to make his return to GL. Maybe he got a new crew together for he purpose of returning and wiping them out.
Bellamy is Dead! Don QijoteFlamingo killed him, for having lost to Luffy. But still I think it should be a swords Man.
^ He MIGHT be dead. I hope he is dead and we won't see him again but he might well be alive if Oda wants him to be, unfortunately.
@Tousen : He is only after ONE of them (maybe Luffy but it's not even sure).
My first guess would have been krieg, but i dont think he would have the capability to get this far on the grand line. Morgan since he was a former marine might be able to. I think it is someone from before the crew entered the grand line.
It could also be one of the CP9, but I think it would be way too soon for a possible comeback...
Also Krieg wasn't that bad, but I don't really see him in this gang.. Maybe it's someone we don't know of after all
I bet it's kuina, looking for zoro
on a more serious note, I'd say arlong is not really possible, because he believes that mermen are better than human, so he wouldn't really be selling his own kind, plus hachi is a former subordinate of his, who has proven that he is loyal to him(arlong). So he wouldn't have his new crew beat up hachi. Krieg is possible, except that he was so pitifully weak I seriously doubt he can make it all the way to merman island, same with morgan. Personally, I think it's kuro because he was planning on living a quiet life(after killing tons of people) and luffy ruined his life. Plus as evil as some of them are, kuro seems like the only one who is a real psycho, while the rest were just evil.
Another palusible suspect would be Spandam, and we know he hates the strawhats. Breaking a glass is no big deal and he can easily express his rage and stuff and probably use military power and that causes everyone to be afraid. His strength is merely his elephant sword but something coulda happened in the very short period during Thriller Bark and the three week period. His body is in aching pain thanks to Robin and thus he has an iron body.