Born from a shell in an underwater society devoid of biological families, Marin considers all sea dwellers kin, sharing an especially sisterly bond with Urin. Her perpetually optimistic and trusting nature leads her to openly express affection, declaring "aishiteru" – a term carrying deep emotional weight – to everyone she meets, including strangers and adversaries, often causing initial cultural discomfort. She wears minimal aquatic attire, later adopting land garments.

Marin's journey begins upon discovering a discarded ring in the ocean, prompting her and Urin to surface for the first time to return it to its owner, Kanon Miyamori. After Urin accidentally unleashes the sealed entity Sedna, Marin is designated the Priestess of the Sea. Tasked with combating encroaching darkness alongside Kanon, the Priestess of the Sky, Marin manifests water-based magical abilities. She primarily uses these powers to purify Sedna-corrupted beings through compassionate physical embraces rather than destruction.

Intensifying conflict brings significant turmoil when Urin succumbs to Sedna's influence, becoming the Priestess of Darkness. Witnessing Urin's corruption and grappling with fear and inadequacy causes Marin to temporarily lose her magical abilities. This despair forces her to confront the complexities of darkness and sorrow, challenging her unwavering optimism. Supported by Kanon, Marin regains her powers by acknowledging and integrating these darker emotions instead of rejecting them. During the final confrontation, learning that Sedna embodies humanity's collective sorrow, Marin advocates for acceptance and reconciliation over annihilation. This approach ultimately frees Urin and peacefully resolves the crisis.

Following these events, Marin returns to the sea, maintaining an implied spiritual connection with Kanon. One year later, in the OVA episode "Shinjiau Kokoro," she continues demonstrating her nurturing nature by helping a lost baby sea cow reunite with its mother.