Kaido, known as Kaido of the Beasts, is a major antagonist recognized as one of the Four Emperors ruling the New World and serves as the Governor-General of the Beasts Pirates. Before forming his own crew, Kaido was an apprentice on the legendary Rocks Pirates, alongside other future powerhouses. His immense power and resilience have earned him the title of the Strongest Creature in the World, a reputation that precedes his staggering bounty of over 4.6 billion berries.

Kaido is a figure of immense physical stature, standing over seven meters tall with a muscular build, long black hair, and a pair of distinctive pointed horns. A large X-shaped scar, inflicted by Kozuki Oden, marks his torso. His personality is famously volatile and erratic, driven by a deep-seated nihilism and a hobby of attempting suicide, stemming from his apparent inability to die. He is a heavy drinker, and his moods swing wildly when intoxicated, cycling through states of tearful melancholy, childish coquettishness, and homicidal rage. Despite this brutish exterior, Kaido possesses a cunning and strategic mind. He used the former shogun Kurozumi Orochi to seize control of Wano Country for its natural fortifications and resources, then systematically oppressed its people through forced labor and manufactured weapons. He also maintains a powerful artificial Zoan army created from SAD produced by Donquixote Doflamingo and Caesar Clown.

Kaido's primary motivation is to instigate the world's most enormous war, believing that the world has become boring and dull. He holds a cynical view of the world's elite and believes that true equality and freedom can only be determined on the battlefield through violence. This desire for conflict is coupled with a deep respect for strength and a sense of honor in combat. He detests interference in his fights and was enraged when a CP0 agent distracted Luffy during their battle, as he felt it cheapened the victory, mirroring a similar incident in his past duel with Oden. He is known for trying to recruit powerful enemies into his crew, including figures like Luffy, Kid, and the samurai Ashura Doji, demonstrating a pragmatic appreciation for strong fighters.

In the story, Kaido serves as the primary antagonist of the Wano Country Arc, where he has established his base on Onigashima. For two decades, he has acted as the island nation's oppressive guardian, or "guardian deity," enriching himself while its citizens suffer. His goal is to create the ultimate pirate paradise called the New Onigashima Project, which involves moving the island and forging a massive alliance. His role as a tyrant is highlighted by his cruel actions, such as forcing his subjects to consume artificial SMILE fruits, which rob them of their ability to express negative emotions while dooming them to eternal, hollow laughter.

Key relationships define his interactions with the world. He holds a business alliance with Big Mom, a relationship built on mutual respect and a shared history as former crewmates, though they are also rivals. His relationship with his son, Yamato, is one of conflict and forced control. Kaido wants Yamato to be the heir to his empire, but Yamato idolizes Kaido's fallen enemy, Kozuki Oden, leading to a violent and strained father-son dynamic. Among his crew, he trusts King implicitly and shows surprising leniency towards his subordinates, even when they fail in their missions, though his drunken rages can turn him against them.

Throughout his development, Kaido is portrayed as a tragic figure who achieved the pinnacle of power but found it hollow. He has been defeated seven times, captured eighteen times, and sentenced to death forty times, yet no execution method could ever kill him. This has left him searching for a worthy opponent who can challenge him, a "Joy Boy" who can finally match his strength and give meaning to his existence. His defeat at the hands of Monkey D. Luffy signifies the end of his reign and the dawn of a new era, though it is framed as a satisfying conclusion for a warrior who finally found a legitimate challenger.

Kaido's notable abilities are centered on his overwhelming physical power and his Mythical Zoan-type Devil Fruit, the Fish-Fish Fruit, Model: Azure Dragon. This fruit allows him to transform into a massive, flying blue dragon or a powerful human-beast hybrid. In his dragon form, he can unleash devastating elemental attacks such as Bolo Breath, a massive fire blast capable of destroying mountains, and a variety of wind-based slashing attacks. He can also create Flame Clouds, which allow him to fly and even levitate entire islands like Onigashima. In combat, he wields a giant iron club named Kanabo, which he uses to execute powerful, lightning-infused strikes like Thunder Bagua. Kaido is a supreme master of all three forms of Haki, including the rare ability to use advanced Conqueror's Haki to infuse his attacks with explosive force, a power few in the world possess. His natural durability is so extreme that his skin is nearly impenetrable, making him a walking catastrophe that can only be injured by attacks that bypass his defenses.