Marin Asagi is the thirteen-year-old protagonist of the anime Brigadoon. She is an orphan who was abandoned as an infant at the doorstep of a tenement house in Yayoi, Tokyo, and was adopted and raised by the elderly couple Gen and Moto Asagi. After Gen's passing, she lives with her adoptive grandmother, Moto, in a close‑knit community of colorful tenants. Marin is a cheerful, energetic junior high school student with a lively imagination and a strong work ethic; she earns her own money by delivering newspapers. Despite her difficult beginnings, she is compassionate and caring toward others, though her imagination can sometimes lead to outlandish fantasies.

Marin's life is turned upside down when a mysterious mirage called Brigadoon appears in the sky, and alien creatures known as Monomakia begin to hunt her. She is saved by a Monomakia named Melan Blue, who becomes her protector. Marin is initially believed to be the Creis—a key biological element needed by Brigadoon for a dangerous ritual called the Pasca. It is later revealed that she is not the Creis herself but carries something similar in her DNA, which makes her a target for both Brigadoon's factions and a Renegade group seeking to disrupt the ritual. This revelation forces Marin to confront the truth about her origins and the reason she is being pursued.

Motivated by her desire to protect her loved ones and her growing bond with Melan, Marin takes on the responsibility of helping to save both Earth and Brigadoon. She is determined, quick‑witted, and brave, often putting herself at risk for others. Throughout the series, she navigates not only alien attacks but also ordinary adolescent challenges: school prejudice, family crises, and strained friendships. Her closest friend is Moe Kisaragi, a quiet classmate who stands by her despite the dangers. Marin also develops a deep emotional attachment to Melan, striving to understand his thoughts and feelings as he evolves from a cold weapon into a more empathetic being.

Marin does not possess superhuman abilities of her own, but her significance lies in her biological link to the Creis and her role as the catalyst for the Pasca ritual. As the story progresses, she receives a transformation‑type Monomakia named Cosmos Yariya, which grants her a combat form to fight alongside Melan. Her greatest strengths are her empathy, resilience, and the trust she builds with those around her. Over the course of the series, Marin grows from an ordinary, hopeful girl into a determined leader who accepts the weight of her fate, forging an unbreakable bond with Melan and ultimately finding the courage to face the collapse of two worlds.