Yūko Tani grew up and was educated alongside Haruo Sakaki aboard the Aratrum. Her grandfather, Daichi Tani, adopted Haruo following his parents' death by Godzilla, forging a familial tie. Despite their shared upbringing, Yūko initially struggled academically compared to Haruo. She developed a complex about her slim physique and physical strength, contrasting with her proficiency and specialization in weapon design and modification.

In *Godzilla: Planet of the Monsters*, Yūko served as a sergeant and weapons specialist. Assigned as Haruo's handler after his release, their initial interactions were formal and strained; she found it difficult to use his first name. During the mission to reclaim Earth, she questioned Haruo about his role in the elderly volunteers' deaths on the Tau-e expedition, indirectly blaming him before retracting her accusation. She applied her technical skills to analyze Servum remains and configure military hardware. Piloting a landing craft during Godzilla's first attack, she witnessed key events, including Eliott Leland's sacrifice. She later joined the power suit team against Godzilla Filius, assisting in implanting EMP probes into its spine. Haruo expressed surprise at her high-risk role, prompting her to reveal her motivation: overcoming physical limitations stemming from her grandfather's death.

Her role expanded in *Godzilla: City on the Edge of Battle*. After crash-landing on Earth, Yūko and survivors encountered the Houtua tribe. She displayed jealousy when the Houtua twins Miana and Maina showed interest in Haruo. Separated from the group, she was attacked by Servum in a swamp and rescued by Haruo and Adam Bindewald. Choosing to remain on Earth to fight Godzilla, she stated she preferred action over living with regret for fallen comrades. Within Mechagodzilla City, she collaborated with Bilusaludo engineer Belu-be to develop and test-pilot the advanced "Vulture" exosuit, noting its performance enhancements despite human limitations. During a private conversation, she confessed her long-standing admiration for Haruo's unwavering determination, contrasting it with the Aratrum's despair. This culminated in her kissing him after he promised to "show her the planet as it once was."

Yūko's involvement reached a critical point during the battle against Godzilla Earth. When nanometal assimilation threatened her and Haruo during Mechagodzilla City's activation, Haruo destroyed the city's core to save her, leaving her comatose. This outcome profoundly impacted Haruo, contributing to his despair and eventual realization about his vengeful quest. She remained incapacitated throughout *Godzilla: The Planet Eater*, her condition a pivotal factor in Haruo's final decision to break the cycle of revenge.

Her relationship with Haruo evolved from a professional military association to a deeper emotional connection, marked by concern for his well-being during reckless actions and her romantic confession. Her technical expertise and resolve in combat highlighted her growth from an insecure subordinate to a committed soldier confronting her physical limitations.