Rio Kastle, known as Rio Kamishiro (神代璃緒 Kamishiro Rio) in the Japanese version, was a human who died in a car accident, with the soul of the Barian Marin continuing Rio's life after her death.
Rio and her twin brother Reginald lived in a mansion with their parents. One night, Reginald was chasing Rio around the house after she stole his Deck of Duel Monsters cards that his father gave him. Their fighting resulted in a suit of armor nearly falling on Rio, but Reginald pushed her out of the way, earning a scar on his right shoulder. One night, Reginald and his family were traveling in a car, which was hit by a truck driven by Chironex, who was fleeing from the police. All four were hospitalized.
Reginald and Rio under medical attention.
All four members of the family died in the hospital. Just after their deaths, the souls of the Barians Nash and Marin were transferred into Reginald and Rio, respectively. This was done by Abyss, after Nash and Marin were killed by Vector. This revived the bodies of both siblings, but stripped Nash and Marin of their memories. Thus, Nash and Marin continued living as Reginald and Rio, with the doctors expressing shock that they were suddenly breathing again.