In the manga of ONE PIECE there was that moment in which Hyo teaches Haki to Luffy in the Udon prison, during the fight that is entertaining Queen, a moment reached recently also in the anime. But as readers know, there is a very particular character in the prison who has decided to use Luffy for his personal strategies.
This pirate is Caribou, the man who had already distinguished himself in the Shabondy archipelago as one of the most dangerous. During the previous sagas of ONE PIECE he had no luck, between the island of the fish men and the first meeting in the mini adventures with Kaido's crew. For this he was also locked up in Udon as the protagonist of the manga.
But during Wanokuni's second act, as you recall, it was useful to knock out all the lumacophones of the island by hiding them in your body thanks to the power of your Devil Fruit. Yet after those moments, although he is certainly not a weak pirate, he was completely sidelined and for now does not seem to have appeared in Onigashima.
Rather, in the entire third phase of this ONE PIECE saga he did not appear. What plans does Oda have for the character? Since it is no longer necessary to keep communications disconnected between Onigashima and Wanokuni, it would be useless to leave the character on the main island and not let him participate in the battles against Kaido's crew. Yet there is no trace of the various ships that have set sail from the docks. He certainly did not appear in the fake ONE PIECE 988 spoilers.