Oto Tachibana, sent by her brother Mikazuchi, infiltrates Yamato Takeru's group to gain trust, steal the Sky Warrior Susanoo, and assassinate Yamato. Her mission falters as she forges genuine bonds with the group and develops romantic feelings for Yamato Takeru. This shift marks her as "contaminated" by her faction, sentencing her to execution.

A critical revelation exposes that Oto is not Mikazuchi's biological sister, taken from her birth mother as a child, a truth Mikazuchi never learns. After Orochi possesses Mikazuchi, Orochi weaponizes the dragon's dark energies to corrupt Oto's soul, establishing a psionic bind to control her mind. This psychic link persists until Orochi's power disrupts Mikazuchi's connection to his mech, freeing Oto.

Following Orochi's defeat, Oto moves to Yamato's village, integrating into the community. While no explicit epilogue details her fate, the narrative strongly implies she pursues a romantic relationship with Yamato Takeru, potentially marrying him.

In the OVA sequel *Yamato Takeru: After War*, set three years later, Oto lives peacefully in Yakumo Village with Yamato and friends. This tranquility shatters when Mikazuchi, piloting Shuranoo, attacks. Oto is abducted to Tsukuyomi, who reveals a parasitic entity used her mother's body as a host. Oto is absorbed into Tsukuyomi's form, while her mother is freed. Yamato Takeru attempts mutual sacrifice, piloting Susanoo with Oto into the sun to destroy Tsukuyomi. Before the point of no return, Mikazuchi intervenes. He approves of their relationship, urging them to live fulfilling lives together, then sacrifices himself by piloting Susanoo into the sun, destroying Tsukuyomi and the mech. Oto and Yamato are safely returned to the Dragon Carrier.