Roronoa Zoro, also known as Pirate Hunter Zoro, is the swordsman and combatant of the Straw Hat Pirates and serves as the first member to join Monkey D. Luffy's crew. He was born in the East Blue and spent his childhood training at the Isshin Dojo in Shimotsuki Village. There, he developed a fierce rivalry with Kuina, the dojo master's daughter, who defeated him over two thousand times in training. After Kuina made him promise that one of them would become the world's greatest swordsman, she died in a fall the following day. In her memory, Zoro took possession of her sword, Wado Ichimonji, and made a solemn vow to achieve that dream for both of them. Before joining Luffy, Zoro worked as a bounty hunter, earning himself the nickname Pirate Hunter throughout the East Blue. He was rescued from execution by Luffy after being captured by the Marines and agreed to join his crew in exchange for his freedom.

Zoro's personality is defined by his serious, stoic, and unwavering demeanor, though he also exhibits a short temper and a comedic, exaggerated side when provoked. He adheres to a strict personal code of honor that resembles bushido, refusing to use trickery or flee from a fight. He is willing to kill his opponents when necessary but never in cold blood, and he shows mercy to weaker foes by using the blunt side of his blades. One of his most persistent running gags is his notoriously terrible sense of direction, which causes him to become lost even in straight lines, a flaw he stubbornly refuses to acknowledge. Despite this, he is often the first crew member to sense danger and react with sharp awareness. He spends most of his time on the ship sleeping or training with weights, and he has a deep fondness for sake.

His primary motivation is to become the world's strongest swordsman, a goal rooted in his promise to Kuina and his desire to honor her lost potential. This ambition drives him to push his physical limits constantly. Beyond personal glory, he is fiercely loyal to Luffy and considers his captain's dream of becoming Pirate King as important as his own. He has stated that if his actions ever dishonor Luffy's goal, he would atone by cutting open his own stomach. This loyalty extends to the entire crew, as he is willing to endure immense suffering and sacrifice his own life to protect them. He once absorbed all of Luffy's pain and fatigue during the Thriller Bark arc, nearly dying in the process, without telling anyone until much later.

Within the story, Zoro acts as the crew's primary combatant and is generally considered its second strongest member, forming the Monster Trio alongside Luffy and Sanji. He typically faces the second most powerful enemy in each major arc. He is also one of the eleven pirates known as the Worst Generation, recognized as a Supernova for reaching the Sabaody Archipelago with a bounty exceeding one hundred million berries. His role as Luffy's right hand has led many characters, including allies and enemies, to mistake him for the crew's actual captain. He is deeply respected for his unwavering will and serves as a moral anchor for the crew, often reminding them of the seriousness of their path.

His key relationships are central to his character. His bond with Luffy is built on absolute trust and mutual respect, and Zoro is one of the few people Luffy allows to challenge his decisions. He maintains a running, often violent rivalry with the cook Sanji, exchanging insults and fighting over trivial matters, yet beneath the bickering lies a deep mutual respect and reliance in battle. His relationship with his childhood rival Kuina, though brief, remains the emotional foundation of his entire journey. During the two-year timeskip, he trained under Dracule Mihawk, the very man he seeks to surpass, which forged a complex bond of mentorship and rivalry between them.

Zoro undergoes significant development throughout the series. Starting as a solitary bounty hunter, he grows into a loyal first mate who understands the weight of his promises and his responsibility to the crew. His defeats, especially his first loss to Mihawk early in the series, humble him and deepen his resolve to become stronger. After the timeskip, he returns with a new scar across his left eye and drastically improved abilities, having learned to use Armament and Observation Haki. His most notable development occurs during the Wano Country Arc, where he awakens Conqueror's Haki and learns to infuse it into his blades, a feat only the greatest swordsmen in the world can achieve. He also acquires the sword Enma, which was once wielded by Kozuki Oden and has the unique property of forcibly draining its user's Haki. Zoro not only masters this dangerous blade but also turns it into a black blade, the greatest achievement in One Piece swordsmanship, marking his ascension toward his ultimate goal.

His notable abilities center on his self-created Three Sword Style, or Santoryu, in which he wields one blade in each hand and a third in his mouth. This style includes a vast array of techniques for close combat, long-range slashing using compressed air, and powerful combination attacks. He can also manifest the Kyutoryu style, also known as Ashura, which temporarily gives him six arms and three heads, allowing him to strike with nine blades simultaneously and tripling his attack power. His Haki mastery includes advanced Armament Haki for hardening his blades and coating them in invisible armor, Observation Haki for sensing enemies and predicting attacks, and Conqueror's Haki, a rare form of willpower that lets him dominate the wills of others and coat his weapons in black lightning. His current set of swords consists of Wado Ichimonji, a O Wazamono grade blade inherited from Kuina that symbolizes his promise; Sandai Kitetsu, a cursed Grade blade that only accepts wielders with sufficient willpower; and Enma, a legendary O Wazamono blade that once wounded the Emperor Kaido and is the instrument through which Zoro continues to develop his power. His current bounty stands at one billion one hundred eleven million berries, reflecting his threat level as one of the most formidable pirates in the New World.