Chapter #998 of One Piece firms up the addition of a new Straw Hat Pirate: Jinbe the Fishman. Jinbe is a familiar face, as he's made multiple appearances throughout the series. As the new helmsman, he becomes the 10th Straw Hat Pirate to join Luffy's crew, but he had a long and storied history before then.
Jinbe is a master of Fishman Karate, and he has a strong sense of justice. Growing up in an orphanage, Jinbe met Arlong, Macro and Fisher Tiger and also attended the Fishman Karate Dojo, graduated with a black belt, and went on to join Neptune's Army about 16 years before the story's current timeline. However, he resigned and joined the Sun Pirates after Fisher Tiger attacked Mary Geoise -- freeing many of the slaves that were there, thus becoming an enemy of the World Government. Jinbe has strong resentment towards the act of slavery and how the Marines overlook it, so that too is a major factor as to why he joined the Sun Pirates.
He later met Koala, a young girl who was formerly a slave. It was while trying to return her to her hometown that pirates were ambushed by the Marines, causing Fisher Tiger to become wounded, later dying from his injury. Jinbe then became a captain in his stead, wanting to continue to fight against oppression and slavery. At some point later in his career, he became one of the Seven Warlords of the Sea, allowing him and his crew to get away with more than other pirates due to their affiliation with the World Government. He saw this as a way to try to strengthen the relations between humans and fishmen, so he accepted.
About two years before the beginning of the series, he met Portgas D. Ace, and the two fought for five days straight before both men collapsed from exhaustion. They had an amicable relationship after Ace was allowed to join the Whitebeard Pirates.
After Ace's capture, Jinbe was imprisoned for refusing to take part in the war against the Whitebeard Pirates. It was here that Ace told Jinbe about how Whitebeard saved Fish-Man Island and stories about his brother, Luffy. Ace asked Jinbe to watch over Luffy after his death, though Jinbe initially refused because Luffy was a stranger and he owed him nothing. After Ace was taken to be transported to Marineford, Luffy and company released Crocodile and Jinbe from their cells, and the group rushed to try and catch up before Ace was executed. It was during the ensuing fight against the Marines that Jinbe resigned from his position as a Warlord. He defended Luffy against Akainu's attacks, and the two later became friends after Jinbe talked Luffy through his grief of Ace's death.
After the time skip, Jinbe and his crew helped Luffy and the Straw Hats defend Fish-Man Island from a coup against the Royal Family by the New Fish-Man Pirates. Luffy lost a lot of blood during the fight, and Jinbe gave some of his in a transfusion. Luffy asked Jinbe to join his crew, but Jinbe declined the offer. However, once his unfinished business was done, he did indeed join. Jinbe then spoke to Neptune about being disassociated from Big Mom, who had taken Fish-Man Island into her territory after Whitebeard's death.
During the Whole Cake Island Arc, Jinbe then attempted to sever his ties with Big Mom but recanted when she threatened the lives of him and his crew in retaliation. It was only when Luffy and his friends went up against Big Mom that Jinbe betrayed her. After the group rescued Sanji, they were pursued by Big Mom and her crew to Cacao Island, where the group was set to meet Luffy and Sanji. When it seemed like Big Mom would overtake the Sunny, the Sun Pirates came and held her back while the Straw Hats escaped.
This brings us to the current Wano Arc, wherein Jinbe once again met up with the Straw Hats and officially joined them as a member. Jinbe has proven to be a valuable ally to Luffy and the others, so he makes a great addition to the crew. He also has a great dynamic with Luffy, so it'll be interesting to see how he influences the young pirate in the coming storylines.
