Nina Tucker is the young daughter of Shou Tucker, a State Alchemist known as the Sewing-Life Alchemist. She lives with her father and her pet dog Alexander in an eastern border city. Her mother left the household two years prior, but in truth the woman was used as an unwilling test subject in her father's chimera experiments and died. Nina is approximately five years old in the manga and the 2009 anime adaptation, and four years old in the 2003 adaptation. She has long brown hair usually worn in braids, blue eyes, and is often seen in a pink polo shirt under a blue dress with bright buttons.

Nina's personality is cheerful, playful, and affectionate. She is outgoing and quickly bonds with others, displaying the trusting nature of a child who craves companionship. Because her father is deeply absorbed in his alchemical research, she often feels lonely and spends most of her time playing with Alexander. She loves her father unconditionally and never stops believing in him, even when his behavior becomes distant and erratic. This blind trust ultimately leads to her tragic fate.

When Edward and Alphonse Elric arrive at the Tucker home to study bio-alchemy, Nina forms a strong attachment to them, referring to both as "big brother" in the manga and 2009 anime. In the 2003 anime, she calls Alphonse "big-big brother" and Edward "little-big brother," a nickname that Edward tolerates without irritation, which is notable given his sensitivity about his height. The Elrics, who have also experienced a distant father and the pain of loss, respond warmly to her and spend much of their time playing with her and Alexander. For a brief period, Nina experiences the family-like closeness she has been missing.

Her role in the story is that of a tragic catalyst. Her father, facing the loss of his State Alchemist certification due to his inability to produce a new talking chimera, uses his daughter and the family dog as subjects for a human transmutation. He fuses Nina and Alexander into a single chimera creature, which retains Nina's consciousness and ability to speak. When the Elrics discover the truth, they are horrified and powerless to reverse the transformation, as human transmutation cannot be undone. Nina, despite her altered form, tries to protect her father from Edward's anger, demonstrating her enduring loyalty. The authorities place Tucker under house arrest, but before he can be taken into custody, the Ishbalan alchemist known as Scar arrives. He kills Shou Tucker and then, recognizing the chimera's suffering, mercy-kills Nina as well.

Nina's death leaves a profound and lasting impact on Edward and Alphonse. Edward is haunted by guilt and nightmares, seeing her face in moments of crisis. He mentions his failure to save "a little girl" when he confronts the Truth at the end of the series, and Alphonse later cites Nina as a constant motivation for them to find ways to use alchemy to help rather than harm. Her tragic story serves as a stark illustration of the dangers of unethical alchemy, the cost of human transmutation, and the betrayal inherent in sacrificing one's own family for ambition. She has no special abilities as a human; as a chimera, she possesses a canine body with the capacity for speech, but that form is a prison from which she cannot escape. Nina's innocence and her eventual suffering become a defining emotional touchstone for the entire narrative, shaping the moral trajectory of the Elric brothers' journey.