Finn Fish appears as a small, angelic being who serves as the mentor and partner to the main character, Maron Kusakabe. Her primary function is to detect demons that hide within works of art, which are corrupting the hearts of humans, and to empower Maron to transform into the phantom thief Jeanne d'Arc to seal these demons away. She is initially presented as a semi-angel, known as a jun-tenshi, whose stated goal is to collect chess pieces that represent God's scattered power. It is claimed that this mission will help God regain strength and allow Finn herself to achieve the status of a full angel.

However, Finn Fish's true nature and motivations are far more complex and deceptive. Despite her small and ostensibly benevolent appearance, she is revealed to be a fallen angel, or da-tenshi, who has been manipulating Maron from the very beginning. Her supposed mission from God is a lie; in reality, the chess pieces she has been collecting are not for God, but for the Devil. Finn is an agent of the Demon Lord, and her purpose is to exploit Maron's powers for the forces of evil, making her a central figure in a divine deception. In the anime adaptation, this betrayal is the result of the Devil kidnapping and brainwashing her, erasing her memories and sending her to Earth as a seemingly normal semi-angel so that God would not suspect her true allegiances. In the manga, her fall is presented as a more conscious choice, born from a tragic past and a promise made to her love interest, Access Time.

Throughout the story, Finn exhibits a personality that shifts between kindness and harshness, largely due to the difficult circumstances of her double life. She presents a friendly and supportive facade to Maron in order to maintain her trust and ensure the mission's success. Conversely, she is often short-tempered and physically aggressive towards Access Time, a dark angel and her rival, who works with Kaitou Sinbad. Finn frequently hits him and rebuffs his advances, claiming she does not like rude men like him. This animosity is a cover for her true, deeply buried feelings, as she secretly loves him. Her complex situation forces her to constantly pretend to be different people, acting as a trusted angel to Maron and a disinterested adversary to Access.

Her role in the story is pivotal as the catalyst for the entire plot, as she is the one who recruits Maron and provides her with the power to transform. As the series reaches its climax, Finn is fully corrupted, becoming the Queen of the Demon Realm and directly opposing Maron as a villain. Ultimately, she experiences a redemption arc. In the anime, she is saved when Maron's genuine love and affection for her break the Devil's brainwashing, restoring her to her true self. In the final battle, she sacrifices herself to protect Maron, dying in the process.

Her development concludes with her reincarnation into a new life. In both the manga and anime, Finn is reborn as Natsuki Nagoya, the daughter of Maron and Chiaki. She is born holding a black earring that once belonged to Access Time, a physical reminder of her past life and the connection she shared with him. Access Time is also reincarnated, and in this new existence, the cycle of their relationship begins anew, with him vowing to marry the reborn Finn in the future.

As a semi-angel, Finn Fish does not possess combat abilities herself but rather acts as a conduit for divine power. Her most notable ability is to grant Maron the power of the reincarnated Jeanne d'Arc, allowing her to transform into Kaitou Jeanne. She is also responsible for sending calling cards to announce Jeanne's heists, a classic phantom thief tradition. In her true form as a fallen angel, she has a much greater command over dark powers and even rules as a queen in the demon realm. Her hair is also a source of holy energy, which in the manga is a coveted and powerful substance.