Masaki Miura is a supporting character in the manga and live-action film Last Quarter, known in Japanese as Kagen no Tsuki. He is introduced as the best friend of Tetsu Sugisaki and a classmate of the elementary school students who become involved in a supernatural mystery. Masaki is an attractive boy from class one who initially catches the attention of Hotaru Shiraishi, one of the main child protagonists, though her infatuation fades as the story progresses.

Masaki comes from a fractured family background. His parents are both actors, and after their divorce, he lives with his father, who is frequently away from home due to work commitments. This upbringing has instilled in him a degree of independence but also a certain emotional distance. He is generally level-headed and logical, preferring to approach problems with rational analysis rather than emotional impulse. However, this pragmatism can come across as curtness; his friend Tetsu Sugisaki notes that Masaki can be dismissive of other people's feelings and sometimes makes tactless comments without fully considering their impact.

Despite his occasionally blunt manner, Masaki is genuinely sincere in his desire to help. When Hotaru, Sae, and Tetsu discover that the ghost of a young woman, whom they nickname Eve, is haunting an abandoned mansion and searching for a person named Adam, Masaki joins the group's investigation without hesitation. He is the one who initially identifies Eve as possibly being Sayaka Kamijo, although Hotaru points out that they do not look alike. Later, working alongside Tetsu, he helps the group correctly identify Eve as Mizuki Mochizuki, a high school student who is in a coma following a car accident. As the mystery deepens and it becomes clear that the spirit possessing Mizuki is actually that of Sayaka, Adam's deceased girlfriend, Masaki uses his logical reasoning to conclude that the only way to free Mizuki is to give Sayaka closure by revealing the truth of Adam's death. This conclusion drives the group's next steps in the story.

Masaki's key relationships are primarily with his best friend Tetsu Sugisaki, with whom he shares a steady camaraderie, and with Hotaru and Sae, who become his companions in the investigation. His interactions with Eve and, by extension, the comatose Mizuki, are motivated by a sincere wish to resolve the supernatural situation rather than by deep personal attachment. Over the course of the narrative, Masaki shows a measured but genuine development: he moves from a somewhat detached and logical observer to an active participant in a compassionate plan to save a stranger's life. He does not possess any supernatural or extraordinary abilities; his primary contributions are his sharp deductive skills, his calm demeanor under pressure, and his capacity to synthesize information into actionable conclusions.