Brook, also known as Soul King and a former member of the Rumbar Pirates, joined the Straw Hats Pirates crew at the end of the Thriller Bark Arc in episode 381 titled “A New Crewmate! The Musician, Humming Brook.”
|Episode Number||Episode 381|
|Episode Name||A New Crewmate! The Musician, Humming Brook|
|Manga Chapter||Chapter 489|
|Chapter Name||The Eighth|
Brook expressed his genuine happiness for being alive to Monkey D. Luffy, the Straw Hat Pirates' captain, despite the difficulties he had faced since coming back to life more than 50 years ago. Luffy agreed to Brook's request to join the Straw Hat Pirates when Brook enquired about the status of his original invitation.
Except for Roronoa Zoro, who was asleep, and Nico Robin, who knew Luffy would accept him regardless of what, all of the Straw Hat Pirates were astonished by this, but they nonetheless welcomed him to the crew. Although it's highly improbable that the Marines would recognize him by his current appearance, Brook thoroughly introduced himself and revealed that he, too, is a wanted pirate with a bounty of 33,000,000 berries.
Usopp and Franky dug a burial for the deceased Rumbar Pirates two days later. Given that the ground at Thriller Bark originates from their homeland, the West Blue, Brook is happy that his friends have a final resting place. He then continued to play the violin for a while. Then Zoro joined Brook in his grief and laid the shattered Yubashiri beside the burial.
The Straw Hats eventually prepared to leave. Franky informed Lola that Brook's ship was ready for use by the Thriller Bark Victims Association. Following that, Lola gave Nami a Vivre Card that directs her to her mother, a famous pirate in the New World (later revealed to be Big Mom, one of the Four Emperors). Luffy pulled out the piece of paper that his brother Portgas D. Ace had given him, which was also a Vivre Card. They were shocked to notice that Ace's card was burning and shrinking, indicating that Ace's life was at stake. Three giant enigmatic figures appeared in the mist, watching over Thriller Bark as Luffy and his crew sailed away. Lola and the other victims made the choice to leave the following day.
Nami questioned Luffy about how he felt about continuing to sail while knowing Ace might be in danger. While some people, like Franky, expressed a desire to accompany Brook directly to Laboon, Brook stated that he didn’t mind a detour. Luffy made the decision to go on, rather than go rescue Ace since he thought that Ace could get out of any danger on his own. He claims that Ace would detest looking weak and wouldn’t want to be saved by his little brother. Luffy then stated that he intended to compete with Ace in the New World at full strength.
Brook revealed his backstory, about how they left Laboon behind and how the Rumbar Pirates met their demise. He also expressed the difficulties he faced in the last fifty years sailing all alone through the sea, but is grateful to be alive and is determined to fulfill his promise to Laboon.
Ace’s Vivre Card was burning indicating that his life was in danger.
A. One of the reasons why Brook joined the Straw Hat Pirates was to fulfill his promise and reunite with Laboon who was waiting for them at the start of the Grand Line.
A. Brook joined the Straw Hat Pirates crew in chapter 489 of the One Piece manga, titled “The Eighth.”