Finn Fisshu is a central character in the narrative, appearing as a small, angelic being who serves as the guide and partner to the protagonist, Maron Kusakabe. Finn is identified as a semi-angel, or jun-tenshi, a being of a lower rank in the celestial hierarchy who aspires to become a Full Angel. In this form, Finn is depicted as having a small stature and white wings, which are characteristic of low-ranking angels in this fictional universe.

In terms of personality, Finn is initially portrayed as kind and devoted to the mission of assisting God. This apparent goodness, however, is later complicated by revelations about Finn's true nature and purpose. Throughout the story, Finn displays a complex range of emotions, from friendly support towards Maron to a gruff and dismissive attitude towards other characters, particularly Access Time. This behavior stems from the difficult and deceptive situation in which Finn is placed, forced to pretend to have different allegiances to different people.

The motivations and background of Finn Fisshu are deeply intertwined with the central mystery of the plot. The stated mission is to help Maron collect chess pieces, which are said to contain scattered pieces of God's power. By doing so, Finn hopes to help God become stronger and achieve the goal of becoming a Full Angel. However, it is ultimately revealed that this mission was a deception. Finn is actually a Fallen Angel who was brainwashed by the Demon Lord and sent to Earth. While pretending to work for God, Finn was in truth manipulating Maron into collecting the chess pieces for the Devil. This makes Finn a tragic figure, an unwitting pawn who betrays the protagonist under the influence of a greater evil.

Finn’s role in the story is primarily that of a catalyst and a guide. It is Finn who informs Maron that she is the reincarnation of Jeanne d’Arc and who grants her the power to transform into the phantom thief Kaitou Jeanne. Through a rosary or similar trinket, Finn lends Maron her own celestial power, enabling her to detect and seal demons hiding within works of art. For much of the series, Finn acts as a mentor and confidante, living in Maron’s apartment and providing information and support during her nightly missions.

Key relationships heavily define Finn’s character arc. The most significant relationship is with Maron Kusakabe, whom Finn initially guides but ultimately betrays under the Demon Lord's control. This betrayal is a pivotal moment, leading to a confrontation where Maron’s declaration of love and forgiveness helps break the brainwashing and return Finn to her true self. Another crucial relationship is with Access Time, a dark angel and semi-angel who serves as Kaitou Sinbad’s partner. Access is in love with Finn, and despite her resentment and harsh treatment of him, calling him rude and a servant of the Demon Lord, she secretly reciprocates his feelings. This unspoken affection forms a key subplot, particularly in the manga where they become a couple before the final battle.

The character of Finn undergoes a dramatic development. She begins as a seemingly benevolent angel, transitions into a brainwashed antagonist and queen of the Demon Realm who betrays Maron, and finally experiences a redemption where she is freed from the Demon Lord's control and reconciles with her partner. The conclusion of her mortal arc is tragic; she is killed during the final battle against the Demon Lord. However, the story provides a form of rebirth. In the manga's epilogue, Maron gives up her powers to allow Finn to reincarnate as a human child. In this new life, Finn is born as Natsuki, the daughter of Maron and Chiaki, while Access is reincarnated as Shinji, the son of Miyako. Natsuki is born with one of Access's earrings, signifying their continued connection, though she does not remember her past life.

Regarding notable abilities, Finn’s primary power is to enable Maron’s transformation into Kaitou Jeanne by lending her divine power. As an angel, Finn can also detect the presence of demons and, being a supernatural being, can communicate telepathically or invisibly with Maron. In her role as a semi-angel, she is small and appears to have the ability to shrink to a very tiny size, often hiding in Maron’s clothing or living unnoticed in her apartment.