Just a theory how Jinbei cans join the SHs
We know that he has a debt on WB and WB is the one protecting the island with his name. But what if WB will fall in the coming war?
Fishmen islans will be attacked once more and Mermaids and fishmen would be put down as slaves once more. Jinbei can't do much alone but he puts his trust on Luffy and starts to travel with him cuz he believe Luffy will be the pirate king and that he will change the world and end the racism against the fishmen. Further more he could strenght himself and at the beginning he could be like a bodyguard for the SHs against stronger foes.
Just a theory :wub: