Mari Sonoda is a central character in the alternate universe depicted in the film Gekijōban Kamen Rider 555: Paradise Lost. In this setting, she is a young woman who originally aspired to become a beautician, a dream she carried from childhood. Orphaned at a young age when a building fire claimed her parents, she was taken in by Hanagata and raised alongside other children at the Ryusei School orphanage, forming deep bonds with her foster siblings. In the film's dystopian world, where the Orphnoch organization Smart Brain has taken over and nearly all of humanity has been killed or transformed, Mari steps forward as a leader of the Human Liberation Army, a resistance group of the few remaining humans. She serves as the group's symbol of hope and morale, tirelessly rallying others to fight against the Orphnoch supremacy. Her personality is marked by a strong will and a tendency to speak her mind directly, never backing down even when facing powerful enemies. Beneath this assertive exterior, she is deeply caring and loyal, especially toward those she trusts, and she shows great resilience in the face of overwhelming despair. She is also skilled at cooking, though her methods are somewhat imprecise.

Mari's primary motivation is the belief that Takumi Inui, the man who can become Kamen Rider Faiz, is humanity's destined savior. Even after Takumi goes missing following a massive attack by the Riotrooper forces, she continues to spread the legend of his return and refuses to abandon hope. When she eventually reunites with a memory-impaired Takumi, she helps him recover his identity, reigniting his resolve to fight. Her role in the story is pivotal as the emotional anchor for the resistance and for Takumi himself. She is captured by the Orphnoch leader Leo, who transforms into Kamen Rider Psyga, and is taken to a coliseum where a crowd of thousands of Orphnoch watch her public execution by a giant Elasmotherium Orphnoch. She is rescued at the last moment by Takumi and by Yuji Kiba, an Orphnoch who initially turns against humanity but later redeems himself.

Key relationships in the film include her unwavering faith in Takumi Inui, her strained cooperation with the cynical Masato Kusaka (Kamen Rider Kaixa), her friendship with the well-meaning but often bumbling Keitaro Kikuchi, and her complicated connection with Yuji Kiba, who ultimately sacrifices his life to save her. Throughout the story, Mari witnesses the death of many comrades yet never flees from the conflict. Her development is shown in her growth from an idealistic believer into a battle-hardened resistance leader who still preserves her core humanity and hope. Though she repeatedly attempts to use the Faiz Driver to transform, she is unable to do so, as the system does not bond with her. Her strength lies not in physical combat power but in her courage, her ability to inspire others, and her unyielding conviction that even in a lost world, a savior can appear and carry humanity forward.