Urin is a sea dweller who lives alongside marine life and regards herself as the younger sister to Marin, a bond formed despite sea people being born from shells and lacking biological families. She has pink hair and green eyes, with an apparent age in her teens. Initially shy, somewhat fearful, and hesitant about the human world, Urin also exhibits a serious demeanor and occasional maturity beyond her years. Underneath this lies a deep-seated jealousy and sense of inadequacy, particularly concerning Marin's attention and affections.

Her pivotal role begins when a ring, discarded by the human girl Kanon, falls into the sea. Urin accompanies Marin to the surface for the first time to return it. During their search on an isolated island, Urin accidentally breaks the seal on a stone sarcophagus, unleashing the dark entity Sedna. This act triggers the central conflict as Sedna threatens the world with enveloping darkness.

Feeling powerless next to Marin and Kanon, who become the Priestesses of the Sea and Sky and gain magical abilities to fight the darkness, Urin's jealousy and negativity intensify. These emotions make her vulnerable to corruption, and Sedna exploits them, ultimately turning Urin into her host body and the Priestess of Darkness. In this role, Urin assists in releasing the sealed sea half of Sedna, fully restoring the entity's power, and declares Marin and Kanon her enemies.

After becoming Sedna's vessel, Urin undergoes a physical transformation, emerging from a cocoon with butterfly wings. She serves as an antagonist for a portion of the narrative, directly opposing the protagonists. The despair caused by her turn to darkness significantly impacts Marin, contributing to a temporary loss of her powers.

Urin's resolution comes when Marin and Kanon, during the final confrontation, discover that Sedna is composed of humanity's collective sorrow and negativity. Instead of destroying her through force, they help the islanders accept and integrate this darkness. This act severs Sedna's influence and frees Urin from corruption. She expresses remorse for her actions and is reconciled with Marin.

Following Sedna's defeat, Urin returns to the sea with Marin, as the people of the sea and the human world are not meant to coexist long-term. A subsequent unaired special episode suggests a continued emotional connection to the events and characters on the island. Another manga adaptation, titled *Umi Monogatari: Minna Aishiteru!*, presents the story in a four-panel comic format and focuses on the everyday experiences of Marin and Urin on the island.