Alvida, also known by her epithet Iron Mace Alvida, is a character who undergoes one of the most dramatic transformations in the series, both in appearance and in her role within the story. She is introduced as the captain of her own pirate crew, the Alvida Pirates, and is the first named pirate antagonist encountered by the protagonist, Monkey D. Luffy. Initially, she is a tall and extremely obese woman with long, wavy black hair and freckles on her cheeks. She commands her ship, the Miss Love Duck, which is adorned with a swan figurehead and heart motifs, reflecting her self-proclaimed status as the most beautiful woman on the seas despite her initial appearance. Her weapon of choice is a large, spiked iron club, which she wields with considerable superhuman strength to terrorize her subordinates and enemies alike. Her early personality is characterized by extreme vanity, cruelty, and a hysterical temperament, as she would brutally punish anyone who failed to acknowledge her supposed beauty.

Her background begins in the East Blue, where her ship served as a prison for a young boy named Koby, whom she forced into servitude as a chore boy. Her initial role in the story is short-lived; after Luffy emerges from a barrel on her ship, he defeats her with a single blow, his Rubber Rubber Pistol, freeing Koby in the process. Following this humiliating defeat, Alvida disappears for a time. She eventually comes across the pirate Buggy, also known as Buggy the Clown, who is adrift and being pursued by a sea creature. She rescues him, and they discover a shared goal of hunting down Luffy, leading them to form a pirate alliance.

This period marks the most significant turning point for Alvida, as she has consumed a Paramecia-type Devil Fruit known as the Sube Sube no Mi, or the Smooth-Smooth Fruit. This fruit has a profound effect on her, causing all of her excess fat to effectively slip off her body. She transforms into a slender, curvaceous, and conventionally beautiful woman with smooth, flawless skin. Her only remaining features from her previous form are her long black hair and her large hat. The fruit's primary ability, however, is not cosmetic. It makes her skin incredibly slippery, causing almost any physical attack or object to slide off her body without causing harm. This grants her a formidable defensive power, making her nearly untouchable in combat against foes who rely on physical blows. She can also use this slipperiness to her advantage in mobility, removing her sandals to use the frictionless soles of her feet to slide across the ground at high speeds, a technique she calls the Sube Sube Spur (or Smooth-Smooth Tracks). Interestingly, despite her dramatic physical change, Alvida seems to be somewhat in denial, once commenting that the fruit only made her freckles disappear.

Her personality also evolves following her transformation. While she retains her immense vanity and confidence in her own beauty, the change grants her a more relaxed, cool-headed, and coquettish demeanor, replacing her earlier, more brutish and hysterical nature. She develops an interest in strong men, notably stating that she likes Luffy because he was the first person to ever strike her "beautiful" face. Her motivations shift from simply plundering villages to seeking revenge on Luffy and gaining power through her alliance with Buggy. She becomes a key member of the Buggy Pirates, often acting as a voice of reason among Buggy's more impulsive followers. For instance, when Buggy's loyal crew wanted to infiltrate the great prison Impel Down to rescue their captain, it was Alvida who persuaded them of the mission's certain death, absorbing his crew into her own in his absence.

Her role in the story continues alongside Buggy. She accompanies him to the town of Loguetown, where they attempt to execute Luffy at the same gallows where Gold Roger was killed. After Buggy is inadvertently elevated to the position of Warlord of the Sea following the Summit War, Alvida becomes an executive in his new mercenary organization, Buggy's Delivery. Her relationship with Buggy is primarily one of convenience, built on a mutual desire for power and a shared grudge against Luffy, though she has also shown envy when Buggy receives treasure. Her development from a minor, comically ugly roadblock for Luffy into a sleek and pragmatic ally of one of the series' recurring characters demonstrates her adaptability. Later in the series, when Buggy forms the Cross Guild alongside Crocodile and Dracule Mihawk, Alvida remains within his faction, continuing to serve as one of his key subordinates. Her notable abilities, therefore, are twofold: her superhuman strength, which she has retained from her days as the Iron Mace, allowing her to wield her massive spiked club, and her slippery body from the Sube Sube no Mi, which makes her almost impervious to slashing and blunt force attacks while granting her unique high-speed sliding mobility.