Of all the characters in Eiichiro Oda's sprawling One Piece world, the bond between Monkey D. Luffy, captain of the Straw Hat Pirates, and Brook, skeleton musician extraordinaire, is special as an example and representation of the series' themes—friendship, loyalty, and the power of dreams.
First Meeting: Thriller Bark Arc
Luffy and Brook's first encounter was during the highly acclaimed One Piece series, specifically the Thriller Bark Arc. In manga chapter 489 and anime episode 381, Brook asks Luffy if he may join the crew, to which he is accepted without a second thought. This seems to be an underlying trait of Luffy, brought about by his intuitive feeling of trust in others and perception beyond appearances.
First off, Brook's skeletal appearance had scared other crew members, such as Nami and Sanji. Still, Luffy's instantaneous acceptance was to set the tone for how Brook would eventually fit into the crew, uniquely placing Luffy in a position where he could see what others couldn't—in this case, through physical appearances.
Brook's Unique Position in the Crew
With Brook being the oldest member among the Straw Hat Pirates, he does hold a special position in the crew dynamics. Oda himself had described the Straw Hat Pirates to be like one big family, and Brook fits into it as the "perverted Grandpa". This is reflected in the attitude Brook took towards Luffy and his remaining crew: respect towards Luffy's leadership blended with a type of grandfatherly affection.
Brook becomes one of the few crew members, which includes Zoro, to be able to treat Luffy both like a friend and as a captain, setting the dual relationship so there wouldn't be a total sacrifice of either aspect—camaraderie or respect, which can go on to further add to the dynamic of the crew.
Shared love of adventure and music
This is a strong foundation of their bond: Luffy and Brook share a very great zeal for adventure and a rush of adrenaline. Brook's musical skills also find a place with Luffy's love for entertainment and revelry, often giving the soundtrack to their crew's adventures and victories.
Brook being the musician of the crew very much fits the bill of what Luffy perceives as the ideal pirate crew. To Luffy, a pirate isn't supposed to be about treasure hunting or ruling powers; it is about freedom and living life to its fullest. Brook's music reverberates with this sense of spirit and makes their journey enjoyable and light.
Loyalty and Support from Brook
Brook's loyalty to Luffy is unshakeable, just like the rest of the Straw Hats. His cheer and perpetual support are a lift in morale. Brook provides wisdom and experience, earned through a really long life and time as captain of the Rumbar Pirates, in times of crisis to Luffy and his crew.
One notable example of Brook's support for Luffy was when he did not flinch from the presence of strong enemies together with his captain. Even though Brook looks comic most of the time, he never falters to stand beside Luffy whenever things get dangerous, proving how loyal and courageous he really is.
The Power of Dreams
Both Luffy and Brook are by strong dreams. The dream of the Pirate King for Luffy corresponds perfectly with Brook's dream of finding his old friend Laboon again, which will satisfy his promise. It is precisely this kind of understanding that makes people know how crucial following one's dreams really is, hence resulting in a great bond between them.
Luffy supporting Brook's dream to see Laboon once again serves as a testament to the kind of captain he is, one who really does care about his crew's personal dreams just about as much as he cares about his own. It is through such mutual support for each other's dreams that their bonds are strengthened and the Straw Hat Pirates grow even closer.
Brook's Role in Luffy's Journey
Some fans and theorists believe Brook plays a key role in Luffy's quest for One Piece. What exactly this is remains speculation, but it gives an idea of how important Brook's knowledge and experience might be to the crew's ultimate goal.
Brook connects to the past by his living in an earlier age of piracy, thus able to provide valuable historical context for the Straw Hat Pirates. This might come in handy in Luffy and companions' journey across the New World, delving deep into mysteries, and hunting down for One Piece.