Miyuki Kodai is the daughter of Susumu Kodai and Yuki Mori. Born to parents serving in the Earth Defense Force, she never spent significant time with either, resulting in a lack of the connection most children have, though she looked up to her mother more than her father.

During the 2220 incident, when Yuki was presumed dead, Miyuki was disappointed in her father, whom she deemed selfish and blamed for her mother's death. She worked as a veterinary intern at a zoo on Earth, specifically at Dr. Sado's animal hospital. During her father's mission to protect the third emigration fleet, she admitted she failed to see why he was an icon when he seemed to care only for the Yamato and saving everyone but her mother.

In the final hour of the evacuation from Earth away from the Cascade Black Hole, Miyuki was placed on a lifeboat by Analyzer on Dr. Sado's orders. She learned Dr. Sado would be staying behind. En route, her lifeboat was struck by lightning and disabled. Hearing it had not reached its destination, Susumu Kodai boarded his Cosmo Zero and searched for the wreckage. He found the ruins, rescued her from the debris, and brought her and other survivors aboard the Yamato. This action helped change her point of view, as she realized her father would do anything to save someone.