Miki Magata is a supporting character introduced as the younger sister of the genius programmer Shiki Magata. She is presented as an adult woman with short black hair and gray eyes, living abroad in the United States with a relative. Her background is defined by her separation from Shiki, who has been confined to a research facility on a remote island since the age of fourteen. This physical and emotional distance means that Miki has largely grown up apart from her sister, and she is initially portrayed as a more socially adjusted and conventional counterpart to Shiki's eccentric and reclusive nature.

Personality-wise, Miki appears calm, composed, and outwardly ordinary, though there is an air of quiet tension about her. Her motivations seem straightforward at first: she is called back to the island during the investigation of Shiki's apparent murder and engages with the protagonists as a concerned family member. However, her true purpose is far more calculated. Miki's role in the story is multifaceted. On the surface, she serves as a narrative foil to Shiki, highlighting the contrast between genius and normalcy. More crucially, her presence on the island provides essential clues about the Magata family history and becomes a key element in the central mystery.

As the investigation delves deeper, it is revealed that Miki Magata is not a real person. Instead, she is an identity created by Shiki Magata herself. Shiki had no younger sister. The character of Miki was a fabrication, a disguise Shiki used to move undetected and orchestrate events from the outside. This revelation dramatically reframes Miki's entire arc: she is not a separate individual undergoing development, but rather a deliberate fiction that Shiki perpetuates to manipulate those around her and eventually achieve her own escape. The supposed family tension and distant relationship between the sisters are part of this deception.

In terms of abilities, Miki Magata as an alias does not possess any unique skills beyond those of an ordinary person. However, the underlying truth that she is actually Shiki means that the character's apparent normalcy is itself a performance, masking the extraordinary intelligence, cunning, and capacity for long-term scheming that characterize Shiki Magata. The Miki persona allows Shiki to interact with the world without revealing her true identity, and its unraveling is central to the story's resolution.