Friendship is one of the top themes shown in shounen manga, and One Piece shows this theme throughout the series, especially in the Straw Hat pirate crew. The most important friendships to note, though, are between Luffy and his crewmates. Let's take a look at the bonds they share:
Luffy and Zoro
Zoro was the first member to join Luffy's crew, and he is a former bounty hunter. After being saved by Luffy from execution by a corrupt marine, Zoro decided to join Luffy on his journey to become the Pirate King. As the series goes on, Zoro and Luffy develop a strong bond. Although Zoro's story may not be as dramatic or tragic as the other Straw Hats' backgrounds, which have helped develop deep, trusting bonds between the members from the start, Zoro does show his loyalty, respect, and trust in his captain. This is shown clearly when Zoro faces Bartholomew Kuma, a Shichibukai or Warlord who comes to Thriller Bark to supposedly annihilate the Straw Hats on the government's order.
Zoro begs Kuma to not capture or harm Luffy, but instead let him take Luffy's place. Kuma, being a secret ally of the Revolutionary Army leader, Monkey D. Dragon, didn't go after Luffy and took Zoro's offer. Kuma tested Zoro's loyalty and Zoro was willing to die to save Luffy and Luffy's dream of becoming the Pirate King. Zoro also shows that he's 100 percent confident in his captain and his capability to achieve his dreams.
In certain arcs in One Piece, Zoro acts more like a friend to Luffy rather than a typical subordinate. For example, in the Water 7 Arc, when Luffy tries to go out and talk to Usopp about coming back to the crew, Zoro stops Luffy and the rest of the crew from going to Usopp. Zoro says that, despite being an idiot, Luffy is the captain and deserves respect. Usopp, who didn't show respect to Luffy and left the crew on top of that, was in the wrong and needed to come to the crew himself. During such times, Zoro talks sense into his captain and the rest of the crew.
This is one of the unique displays of friendship that's not usual for a pirate crew. Most pirate crews in One Piece show a clear captain-subordinate relationship. But, in the Straw Hat crew, Luffy treats his members as equals and friends rather than subordinates. Of course, as the captain, his orders are absolute, and the members comply to them. But the catch is that they take his orders because they believe in him. The crew members' core values are shared, and that's why everyone gets along so well.
Luffy and Usopp
Luffy and Usopp have an interesting relationship that was once strained. After Luffy, Zoro, and Nami help Usopp's village from a money-hungry pirate, Usopp joins the crew, sometimes joking that he's the captain of the ship. In the Water 7 Arc, Usopp leaves the crew after Luffy decides to get a new ship. The Going Merry was the Straw Hats' ship, which was given to them by Usopp's friend, Kaya. However, the ship wasn't in great condition and the ship couldn't travel anymore.
Because of Usopp's strong, emotional attachment to the Going Merry, he decides to leave the crew, thinking that Luffy was abandoning the Going Merry and thought that weak members shouldn't be part of the crew. However, Usopp joins the battle at Enies Lobby, strengthening his bonds with his crew unintentionally and eventually joins the crew again after apologizing to Luffy. If Usopp hadn't been on Enies Lobby to save Robin like the rest of the crew, he most likely wouldn't be part of the crew after they left Water 7. After this incident, Usopp changes and shows his maturity from the entire incident; for example, he doesn't call himself captain anymore while on the ship and shows his respect and loyalty to Luffy once again.
Luffy and Sanji
Picture Caption: Luffy pretending to be Sanji
Luffy and Sanji's meeting was quite simple, but they bonded through a battle against Don Krieg and his men as well as their desire to fulfill their dreams. Sanji wants to find the mysterious, legendary sea, All Blue. Sanji decides to venture out to sea and joins Luffy's crew to find All Blue.
Sanji shows his loyalty to his captain in various ways. When Kuma came after Luffy and the Straw Hat crew on Thriller Bark, Sanji wants to take Zoro's place. He says that he may not have caught the marine's eyes yet, but he'll be a dangerous person one day; Kuma should take care of him before it's too late. But Zoro shoves Sanji aside and becomes the sacrifice to save Luffy and his dreams. However, Sanji does show his loyalty and respect in his captain in this scene.
Another scene that displays the friendship that the Straw Hats have amongst themselves is when Sanji doesn't hesitate to punch Luffy when Luffy was about to say something irreversible to Usopp. Sanji shows that he can do what needs to be done for the crew, but he also does what he can as a friend.
Luffy and Nami
Luffy and Nami have brother-and-sister relationship. Their association started out with Nami traveling with Luffy and Zoro. But Nami stole their ship after the Baratie Arc and used it to get back to her home village. Nami is then revealed to be a member of Arlong's crew and a thief. However, the truth is later known, thanks to Nojiko, Nami's sister. Nami is actually stealing money and treasure to save her village from Arlong.
But when Arlong goes back on his word and uses dirty tricks to keep Nami in his crew, Nami gets frustrated and angry, even going so far as to stab the tattoo on her arm that proves she's in Arlong's crew. Luffy stops her and she asks him for help, which already shows that Nami trusts Luffy somewhat. Luffy entrusts his straw hat, which he calls his treasure, to Nami, and this shows Nami that Luffy trusts her, too.
After defeating Arlong and his crew and saving the village, Nami joins Luffy's crew. Her loyalty and respect for him shows throughout the show, especially when the Straw Hats get separated and she decides to stay on Weatheria to learn about the weather. Since she's a navigator, Nami wants to draw a map of the whole world and help Luffy reach his dreams with her navigation skills. Their dreams and Luffy going the extra mile to show that he wants Nami as part of his crew is what creates such a strong bond between these two members.
Luffy and Chopper
Chopper is the adorable talking reindeer doctor who's from Drum Island. Luffy and Chopper met when Nami was taken in by the doctor, Doctorine, and Chopper helped heal Luffy, Sanji, and Nami. Luffy thinks that Chopper is an interesting and neat creature, but Chopper has a complex about his appearance and his unique traits. Chopper was once rejected by the people of Drum Island and his own reindeer family for his appearance and treated like a monster. This is what developed Chopper's wariness around humans, especially around Luffy's crew.
However, after saving Drum Island from Wapol, Luffy basically commands Chopper to join his crew and become a pirate. Chopper decides to go out to sea with Luffy. Although Chopper was insecure with himself, Luffy likes him just the way he is and the rest of the crew accepts Chopper as he is, too. Chopper's bond with Luffy starts with Luffy asking him to be the doctor on his ship, surprising Chopper with his easy acceptance of Chopper and his appearance. Chopper admires Luffy in many ways and respects his captain.
When the Straw Hats got separated on Sabaody, Chopper stays on the island he was sent to and vows to become a strong fighter and better doctor for Luffy. During the Fishman Island Arc, Chopper shows that he's not afraid to turn into a monster if it's for Luffy's sake. This further proves that Chopper has a lot of respect for his captain and has overcome his complex about his appearance to become a stronger individual for Luffy.
Luffy and Robin
Robin joins Luffy's crew after the Alabasta Arc and after Robin had nowhere else to go. She was once an assassin and worked with Crocodile. However, she didn't show any loyalty to him because he was after the Poneglyphs for the wrong reasons.
After Ohara was destroyed, Robin continued running from the government after they declared her existence was dangerous and joining pirate crew after pirate crew just to survive. However, after meeting and traveling with the Straw Hats, Robin changed and even sacrificed herself to save them during the Water 7 Arc.
Luffy and Robin's bond starts from the point when Luffy saved Robin from the falling rubble in the underground passage, and it grows from there. When Luffy proves that he's on Robin's side, even ordering Usopp/Sogeking to shoot down the World Government flag, he demands that she says she wants to live. When Robin declares for the first time that she wants to live, she fights for her life to go back to her crew's side, displaying even more love, respect, and loyalty to her captain and crew.
Luffy and Brook
During the Thriller Bark Arc, Brook is seen on an empty ship. Luffy finds him interesting and wants Brook to join his crew. The Straw Hats fight Moriah's minions alongside Brook and eventually find out that Brook has a goal: to go back to the whale, Laboon, waiting at Reverse Mountain. Brook finds out that Luffy and the Straw Hats befriended the whale and they can take Brook to him after their adventures. Surprised and ecstatic about being alive, Brook decides to join the crew as their musician and swordsman.
Brook and Luffy's bond starts almost immediately when they first meet and grows stronger during the Thriller Bark Arc. Thanks to Luffy's invitation and his fateful encounter with Laboon, Brook will never be alone again or in darkness. Later on in the series, when Brook becomes a famous singer, Soul King, he abandons his stardom and joins the crew again, showing his deep loyalty to Luffy and the Straw Hat crew.
Luffy and Franky
Although Luffy and Franky started off as enemies during the Water 7 Arc, they become friends later in the arc. Franky fights alongside the Straw Hats during the Enies Lobby Arc, even risking his life to save Robin. Through the battles and obstacles they go through during this arc, Franky comes to admire the Straw Hats and builds a new ship for them called the Thousand Sunny. After some struggle, Franky finally joins the crew and becomes the shipwright and cyborg.
As you can tell from the background stories shared here, Luffy created his bonds with his crewmates through some trials and tribulations. Dreams are also part of the connections he has with his members, and Luffy vows to become stronger than anyone to protect his friends. He never calls any of his members subordinates, even though he expects them to listen to his orders. Even outside of his crew, Luffy treats people he likes as his friends. Luffy is the symbol of friendship in One Piece. He has the ability to attract allies, which Dracule “Hawk-Eye” Mihawk observed during the Marineford Arc. With Luffy as the main character, One Piece portrays true friendship through Luffy, making friendship one of the most consistent and most used themes in this series.