Amelia Ann McFly entered the world roughly a year after her father, the notorious thief Gasback, pulled off a major bank heist. Her mother, rejecting Gasback's criminal life, stayed in their hometown with money and a metal box holding his "precious secret." Despite these provisions, rival thieves plundered their home, and neighbors shunned the family of a criminal. When Amelia's mother fell ill, doctors refused treatment, leading to her death. Amelia blamed Gasback for this loss, igniting a burning desire for vengeance. At nineteen, she journeyed to Macca City with one goal: find and kill him.

Amelia possesses long maroon hair and blue eyes. Her attire consistently features a cowboy hat, a tan brown long coat, a loose light violet or white blouse beneath a black corset marked by an "X" stitch, and magenta jeans secured with double belts. Her build reflects practical combat readiness.

Characterized by anger and reserve, Amelia exhibits a blunt demeanor with scant tolerance for others, especially lecherous men. This aversion stems from acute contact dermatitis triggered by skin contact with men, which she calls an allergic reaction to "idiots." Her singular focus on revenge makes her dismissive of connection. However, exposure to Vash the Stampede's altruism gradually softens her, revealing deeper emotion and vulnerability. Her development culminates in shifting from vengeful aggression to embracing non-lethal resolution.

Her Macca City journey starts aboard a Sandsteamer where she encounters Vash. After he intervenes in her fight with male bounty hunters, she expresses gratitude but remains irritated by his flirtations. Later, during a tavern outing with Vash, Meryl Stryfe, and Milly Thompson, her intoxication sparks a bar brawl. Vash ensures her safety, carrying her back to their hotel, where she recounts her past and motives. She initially condemns Vash for saving Gasback twenty years prior, believing it caused widespread suffering.

Amelia confronts Gasback at Mayor Kepler’s mansion, intent on killing him. Vash interferes to prevent violence, prompting her to punch him furiously before pursuing Gasback alone. Gasback escapes after stealing the city’s power plant, plunging Macca City into chaos. Amelia joins the chase alongside other bounty hunters. When Vash is shot and apparently sinks into quicksand, his presumed death forces her to confront the consequences of her actions. She realizes her revenge quest endangered innocents, acknowledging she is "no better than Gasback," reshaping her perspective.

Teaming with Nicholas D. Wolfwood, Amelia raids Gasback’s hideout to recover the plant. After Vash reappears and subdues Gasback, Amelia counters Gasback’s EMP weapon using a mechanical glove. He recognizes it as one he crafted for his wife, leading Amelia to reveal she is his daughter. Though she aims her rifle at him, she chooses not to kill, adopting Vash’s belief in life's value. After returning the plant to Macca City, she bids Vash farewell, expressing her intent to follow his principles.

In combat, Amelia employs a unique magazine-fed lever-action rifle modified into a "Mare’s Leg" configuration, functioning as a pistol or braced weapon. She favors evasion and kicks in close quarters, minimizing physical contact due to her dermatitis. Her rifle blends elements of Browning BLR and M14 designs, firing semi-automatically with the lever primarily used for bolt manipulation.