Maki Nikaido is an elementary school-aged girl who becomes a central figure in the events surrounding a deadly man-made virus. She is the daughter of Dr. Kimihiko Nikaido, a scientist who was involved in the creation of a pathogen engineered to be a devastating bio-weapon. Her appearance is marked by short, chin-length black hair and brown eyes, and she is frequently seen carrying a patchwork teddy bear that contains a recording of her mother's voice.

Maki's background involves deep family tragedy, as she witnessed the gruesome death of her father at the hands of his assistant, Dr. Kimiko Kujo, who is the leader of an eco-terrorist group. In his final moments, Maki's father injected himself with the virus and destroyed the only antidote to prevent it from falling into the wrong hands, sending Maki to find the detective known as L with crucial research data. This traumatic event is the primary driver of her motivations throughout the story, which are initially focused on survival and later consumed by a desire for revenge against those who murdered her father. In the novelization of the film, her personality is more developed; she is described as somewhat of a troublemaker, bold, and protective of those she cares about, and she speaks in an Osaka dialect.

In the story, Maki serves as one of the children L takes under his protection during his final days. She seeks out L after her father's death, bringing with her the threat of the virus. Her role becomes even more perilous when, during a confrontation with the terrorist group at L's headquarters, she injects herself with the virus in an act of defiance, turning her own body into a ticking time bomb and a carrier of the pathogen. This act forces L into a race against time to find a scientist who can recreate the antidote before the virus becomes active and before L's own life runs out. Her desperation leads her to briefly attempt to take revenge on Kujo, a choice she ultimately steps back from after L's intervention and faith in her.

Her key relationships are central to the narrative. She forms a close, almost paternal bond with L, who, despite his awkward demeanor and unusual habits, shows her great care and protection. She also interacts with the young mute boy who becomes known as Near, and the three form an unconventional family unit on the run. The FBI agent Suruga also becomes involved in her protection. Through her trauma and the time spent with L, Maki undergoes significant development. She begins as a traumatized victim, then becomes someone driven by vengeance, but ultimately chooses to let go of her revenge, demonstrating her capacity for healing and humanity. Her notable abilities are not physical or intellectual in a detective sense, but rather lie in her personal resolve and bravery, such as her decision to inject herself with a deadly virus to ensure a sample is preserved.