Bia is the fifteen-year-old protagonist of the series. She is a young witch from the Kingdom of Magic who is sent to Earth to participate in a contest to determine the next queen of the witches. Her task is to live among humans and learn about their behavior, with the winner of the competition between her and her rival being crowned the new ruler of the Witch Kingdom. Upon arriving on Earth with a small umbrella, she is welcomed into the Giappo family. The mother of the household, Mami, is a former witch who uses a spell to make her husband and two children believe that Bia is their eldest daughter returning home after a long trip abroad.

Bia has a vibrant and freewheeling personality, described as cheerful, sunny, and full of energy. She is often portrayed as a troublemaker who can be selfish and impulsive, frequently using her magic for personal gain or to get out of awkward situations. Despite these flaws, she has a strong sense of justice, cares deeply for her new family, and becomes increasingly attached to the human world as the series progresses. Her initial motivation is to win the throne, but she also seeks to understand the strange behaviors and emotions of human beings, which are foreign to her at first. As she experiences friendship, family conflicts, and her first romantic feelings, her goals shift from simply winning a competition to genuinely connecting with the people around her.

In her role within the story, Bia is a magical girl in training who must balance her secret identity with her daily life as a student. She attends school, makes friends, and deals with typical teenage issues, all while competing against Noa and fending off magical interference from the clumsy guardian wizard Ciosa. Her primary relationship is with Noa, her cold and calculating rival who initially despises humans and will use any means to win the throne. Despite their intense rivalry and Noa's early attempts to harm Bia, Noa occasionally saves Bia from danger to keep their competition fair, and over time she grows to respect Bia and appreciate human beings, becoming a loyal opponent. Within the Giappo household, Bia lives with her adoptive mother Mami, her mischievous younger brother Rabi who often plays pranks on her, and her younger sister Apo who, despite her innocent appearance, wields a large rubber hammer to protect herself from those who harm her. Bia also experiences a short but meaningful romance with Dany Hamilton, a blond tennis player who helps her learn the meaning of romantic love.

Bia undergoes significant development throughout the series. When she first arrives, she clashes with human behavior and feels alienated from the family, leading her to run away. However, after Mr. Giappo rescues her from drowning and the family shows her genuine care and repentance for their earlier coldness, she begins to understand human emotions and become attached to them. She matures from a self-centered brat into a more giving and thoughtful person, learning valuable lessons about the importance of friends and family. At the end of the series, both Bia and Noa end their contest in a tie, and while Noa is disqualified for not understanding humans, Bia is sent back to Earth to continue her training because she has developed too many human feelings.

Bia performs magic using a heart-shaped pendant, activating its power by kissing her fingers and uttering the magic word "scialanda". She is a capable witch, though not as immediately powerful as Noa, who possesses a magic stone on her forehead that grants her enormous strength. Her magical abilities allow her to transform objects, create items out of thin air, and alter perceptions. The series is also notable for its significant use of fan service, frequently depicting Bia in her underwear or a transparent nightgown and placing her in embarrassing situations, which was a distinguishing characteristic of the show at the time.