Mutsuki Saegusa is a secondary but pivotal character in the anime Interlude. She is portrayed as a shy girl who attends the second year of middle school. Her quiet and reserved nature defines her initial presence, as she keeps largely to herself and is not directly involved in the everyday events that the main characters experience. Her background is closely tied to her older sister, Miyako Saegusa, and her brother-in-law, Ikuo Fuyuki, both of whom are central figures in the Pandora Project. Through this family connection, Mutsuki becomes unwittingly entangled in a large-scale experiment aimed at preserving human consciousness after the collapse of reality. Her primary motivation is not overtly expressed, as she exists largely in a passive state, but her role is defined by her subconscious: her dreams form the foundation of the moon city, a constructed dream world designed to isolate and protect humanity from the crumbling real world. She is the dreamer whose sleep sustains this artificial realm, making her the linchpin of the entire Pandora Project. Her key relationships are familial rather than romantic or adversarial. She is linked to her sister Miyako, who appears to be involved in the project, and to Ikuo Fuyuki, the project leader who orchestrates the use of her dream state. These connections place her at the center of the story's conflict without requiring her to act directly. Throughout the narrative, Mutsuki does not undergo a conventional character arc or display notable combat or intellectual abilities. Her development is more symbolic than personal: she transitions from an ordinary shy schoolgirl to a figure of immense existential importance, as the stability of the constructed world depends entirely on her uninterrupted dreaming. Her notable ability is not a skill she controls consciously but an inherent trait: her dreams have the power to house an entire alternate reality. This ability is the key to the Pandora Project and the central mechanism that keeps the moon city from collapsing into the real world. Mutsuki Saegusa remains a quiet and largely passive character, but her role is indispensable to the plot, as she is the silent foundation upon which the story's reality is built.