Finn Fish presents herself as a small, cheerful angel who serves as the guide and partner to the protagonist Maron Kusakabe. However, her true nature is far more complex, as she is secretly a fallen angel who has been manipulating events for her own purposes and on behalf of the Devil. Her outward appearance is that of a tiny, fairy-like being with green hair and eyes, but she is capable of manifesting in a larger, human-sized form.

Her background is tragic and morally intricate. Originally a sickly human girl, she was killed by her own brother, who sought to use her holy power for profit. After this death, she became an angel. Driven by anguish over her brother's betrayal, she killed him in retaliation, which was a grave taboo for an angel. This act led to her being cast out and becoming a fallen angel. The devil took advantage of her situation, and she pledged herself to his service. In the anime adaptation, this fall is depicted as a result of being kidnapped and brainwashed by the Devil, whereas in the manga, her descent is a more voluntary act driven by a promise made to another angel, Access.

Initially, Finn appears to be a kind and loyal assistant, enthusiastically helping Maron detect demons and granting her the power to transform into the phantom thief Jeanne. Her stated motivation is to collect chess pieces containing God's power to help Him grow stronger and to achieve her dream of becoming a full angel. This facade of friendliness is a calculated performance to earn Maron's trust. Her true motivation is to gather these same chess pieces for the Devil, whom she serves as his "Queen". Her personality is thus a duality: sweet and comforting to Maron, but gruff and dismissive, especially towards Access, whose interference she does not appreciate.

In the narrative, Finn acts as the primary catalyst for the entire plot. She is the one who selects Maron, a lonely high school student and the reincarnation of Joan of Arc, to be her earthly agent. Throughout the story, Finn provides Maron with missions and encouragement, all while secretly ensuring the chess pieces are delivered to the Devil. Her most significant relationship is with Access, the angel partner of Maron's rival, Kaitou Sinbad. Despite her harsh treatment of him, she secretly loves Access, and this love is a central driver of her actions. Her relationship with Maron is built on deception, but it evolves. After the truth is revealed and a confrontation occurs, Finn admits that she has genuine care for Maron, ultimately sacrificing herself to protect her from the Devil.

Finn undergoes a significant character arc, transforming from a trusted ally to a revealed antagonist and finally to a redeemed figure. After her betrayal is exposed, she becomes an enemy, but Maron's refusal to fight her and her expression of love for Finn help break the Devil's hold on her, allowing Finn to return to her true self. In the manga's conclusion, Finn dies from her injuries during the final battle. However, Maron uses her power to allow Finn to be reincarnated as Natsuki, the future daughter of Maron and Chiaki, where she is born holding Access's black earring as a token of their past connection. As an angel, her notable abilities include detecting demonic presences, lending her holy power to transform Maron into Kaitou Jeanne, and, in her true fallen angel form, she possesses considerably greater power.