Marine Amagi drives the narrative of Tantei Kageki Milky Holmes TD as a super idol. The theft of her signature song, "Kiseki no Uta," during a concert results in the loss of her seven sentient musical abilities, the Elements, core to her performances and idol identity. This incident sparks the main plot, leading her to seek help from the Milky Holmes detective team to recover the scattered Elements.

These Elements embody distinct musical concepts with unique personalities: Melodia (vocal tenderness), who nearly got stolen; Harmony (musical concord); Shout (rock spirit, loud expression); Rhythmic (tempo); Dancing (boyish demeanor, focus on dance); Decrescendo and Crescendo (decreasing/increasing volume, sharing archaic speech patterns); and Amore (affectionate performance). Each was retrieved sequentially by Marine and Milky Holmes during their investigation.

Marine's background reveals emotional history tied to her idol career. Her interactions with fellow idols Alice Myoujinagawa and Opera Kobayashi surface, highlighting past collaborations and underlying tensions, suggesting professional rivalries or unresolved personal dynamics. Losing her Elements forces her to confront vulnerabilities concerning her artistic identity and legacy, moving beyond her public persona. Her journey involves reconciling these personal and professional challenges while collaborating with Milky Holmes.

Throughout the recovery, Marine displays both determination and emotional fragility. Her reliance on Milky Holmes contrasts with her established status, revealing layers of pressure and expectation. While all Elements are successfully retrieved, her character development emphasizes internal growth through the crisis, not a fundamental transformation of her core traits or public role.