Bellamy fits well with franky and Usoop. They three were betrayed by someone of trust (Kuro, Lucci, Doflamingo) Their enemies lied and the whole island believed them (Kuro work in the island, lucci work in the island, dolamingo as the king). They three were the captain of a gag and they had a ship story. Now Bellamy is learning doing flags (this is a idea that i saw in @Tokiro Oumaga theory) and luffy doesnt know how to make flags. Of course that ussop can make flags, but he also know how to fix a ship but someone better appeared to take that role, franky the ship master and bellamy maybe the flag master. They three had a bad thing in the face. Bellamy scar, Ussop nose, Franky triple chin jajaja. They also were in once time an enemy of luffy. Bellamy always had a dream, the esmerald town and being in a famous pirate crew, so he had something to complete. Remember that all the fleet of luffy accepted being part of the fleet, but bellamy doesnt, and he took the vivre card by some reason and he had to do something before. Then we saw him in a cover learning flags and drawing luffy crest. For me this is so strange