One Piece has a famously colorful cast, especially the individualistic Straw Hat Pirates. The crew assembled from different parts of the world and each member has something unique to offer, be it a skill or characteristic. Interestingly, one of the details they have in common is a tragic past. Each Straw Hat has a tear-jerking background story, but the saddest one of all concerns their skeleton musician, Brook -- and it doesn't seem like his future is that bright either.
Luffy invited Brook to join the crew primarily because he was a talking skeleton, much to his crewmates' annoyance. Even though his entrance wasn't traditional, he sailed through the seas and improved along with the other Straw Hats. Brook's humor and music livened the crew on multiple occasions, though his jokes never seem to get through to Robin. His life, however, wasn't always so joyous.
Before Brook became the musician of Luffy's gang, he was a member of the Rumbar Pirates, a crew made up of music lovers. Their first captain, Yorki, and the other Rumbar Pirates were the ones who promised One Piece's most beloved whale, Laboon, that they would return after circumnavigating the world.
Everything was going well and Yorki even gained a bounty, but he and other members of his crew eventually got sick. In a bid to save the uninfected, the captain sent Brook and the healthy members to continue their journey across Grand Line as the sick departed the path through Calm Belt. As the healthy members continued, Brook was appointed as the second captain.
The renewed Rumbar Pirates' journey was smooth and Brook also earned a bounty, to the delight of the crew. However, another tragedy struck them in Florian Triangle when they were attacked and poisoned by a fellow pirate crew. Knowing they had the Revive-Revive Fruit, they decided to have one member consume it and fulfill their original promise to Laboon. Brook was the (un)fortunate soul, and suggested they record one final song so he could play it to Laboon when they eventually met again. Thus, the crew mustered every last bit of their strength to play Bink's Sake one final time, and Brook watched the others perish one by one as they played before dying himself.
Brook's life prior to meeting Luffy and the others wasn't the only sad aspect of his life. While his fellow Straw Hats have grand dreams like being the Pirate King or becoming the strongest swordsman, Brook's dream is simply to fulfill a promise to an old friend who has been waiting for years. Assuming all of them will eventually realize their dreams by the end of One Piece, Brook will have to relive the saddest moment in his life as he plays what they've recorded to Laboon -- and it doesn't even end there.
Just like any other Devil Fruit in One Piece, Brook's Revive-Revive Fruit gives him supernatural powers. It not only allows him to revive once, but also makes him biologically immortal. Since he was already reduced to bones, he can hypothetically survive as long as he wants, assuming his skeleton isn't eradicated before that. This gives him more than enough time to see his own dream come true and witness the other Straw Hats accomplish theirs. But once that's done, it would be a repeat of his first life -- waiting and watching his beloved crewmates perish before him.
The Straw Hat Pirates' resident musician may have a jolly personality and loves making (often perverse) jokes, but he also carries the saddest story of them all. Brook's past was filled with bitterness, and even achieving his dream may still be bittersweet. The good thing, however, is that it may not be as sad as it seems, since he hopefully won't have any more regrets by then.