WARNING: The following contains spoilers for One Piece Chapter #1018, "Jimbei vs. Who's-Who," by Eiichiro Oda, available in English via Manga Plus and Viz Media.
The past couple of chapters of One Piece have given readers more insight into the history of Who's-Who, one of Kaido's Tobiroppo. During his showdown with Jimbei in Chapter #1018, Who's-Who opens up about his past and the struggles he faced before becoming one of the most powerful Beast Pirate members. Most surprisingly, we also discover that his past intertwines with that of Luffy and Shanks.
12-years before the raid on Onigashima, Who's-Who was an agent working for a secret World Government organization called CP-9, an assassin group made up of users of the Rokushiki martial arts style. However, his status as a CP-9 member was stripped when the ship he was on was attacked by Shank's crew and the Gomu Gomu no Mi Devil Fruit they were transporting was stolen. As a result of his failure, Who's-Who was imprisoned.
While incarcerated, he was relentlessly tortured. But one of the guards in charge of watching him told him stories of the Sun God, Nika. The legend was popular among slaves, as it was said that Nika would fight for the freedom of those enslaved. The guard told Who's-Who that he should pray to Nika for salvation from his imprisonment, and while this advice was probably half-hearted, Who's-Who clung to it. This made him look into the history of the Fish-Men in greater depth, who historically had been enslaved more than most other races. He also became highly interested in the Sun Pirates, a crew made up of former slaves that Jimbei once belonged to before joining the Straw Hats.
A few days after the guard told Jimbei about Nika, he disappeared. This caused Who's-Who to believe that it was because of the legend and that he would be next. It also prompted him to make his escape attempt, which was successful. Sometime after escaping prison, he encountered Jimbei, though not directly, and went on to captain his own crew of pirates. Sometime later, he joined the Beast Pirates as a member of the Tobiroppo, which is when he began plotting to take the spot of one of the current All-Stars.
As a result of the theft of the Gomu Gomu no Mi, Who's-Who became embittered towards Shanks. However, once he learned that Luffy had eaten the Fruit and inherited Shanks' Straw Hat, he decided that Luffy would be the new target of his vengeance. When he heard Jimebi declare himself helmsman of the Straw Hats, the bitter feelings that Who's-Who felt towards Shanks and Luffy came back.
During his recent fight with the Fish-Man in One Piece Chapter #1018, he asks if the legends about Nika are true, believing that Jimbei's people will know the answer considering their history. This only angers Jimbei, however, who scolds Who's-Who for having such racist views about his people. As the fight continues, he pushes Jimbei to tell him everything he knows about the legends of Nika. He never learns the truth, however, as Jimbei has seemingly defeated him before revealing much. It's unclear whether we've seen the last of Who's-Who yet, but this probably won't be the last we hear, at least, of his surprisingly long history with Shanks, Luffy or the Gomu Gomu no Mi.