Apo Zapo is the younger sister of Rabi and the adoptive sister of the series' protagonist, Meg. She is one of the two biological children of Mami Kanzaki, a former witch who gave up her claim to the throne to marry a mortal man. When Meg arrives on Earth to begin her training, Mami uses magic to bewitch her husband and children, making them believe that Meg has always been the eldest child in the family.

Apo is the youngest member of the Kanzaki household, often identified by her blonde hair styled in girlish pigtails. In terms of personality, she initially fills the role of an annoying younger sibling to Meg. Alongside her brother Rabi, she often teases and causes trouble for Meg, adding to the chaos of their daily life. However, compared to her brother, Apo demonstrates a capacity for growth. As the series progresses, she becomes less of a nuisance and shows a more innocent and morally grounded side. She is notably opposed to Rabi's more inappropriate or mischievous behavior and is not afraid to express her disapproval, sometimes taking direct action by hitting her brother with objects when she feels he has crossed a line. Despite her small stature, she possesses surprising physical strength.

Apo's primary role in the story is as a member of Meg's human family, providing a connection to the everyday world that Meg is learning to navigate. Her relationship with Meg evolves from that of a simple annoyance to a genuine sibling bond. As Meg matures and learns about human emotions like compassion and self-sacrifice, Apo comes to see her as a cool big sister. This familial connection is powerful enough to persist even through magical interference, as shown in the final episodes when Mami attempts to erase the family's memories of Meg, who must return to the witch world. While the father is left with only a vague sense of loss, Rabi and Apo retain their memories of Meg and insist on searching for her, demonstrating the deep and authentic love that developed between the sisters.