Ryoga's younger twin sister. Rio once dueled IV who was operating under the orders of Tron. He played a card, "Flaming Hell Blessing", that set the surrounding area on fire. She was critically injured in the blaze and remained in the hospital for the entire first series. After the incident, Ryoga swore to avenge his younger sister, aiming his hatred towards IV.
She recovers in the second series and is able to return to school again. Rio stays strong and prefers to defend herself for her brother Ryoga, fearing his many enemies will use her to get to him. She is a confident and adamant person, getting annoyed when things do not go her way. She is also a skilled duelist who uses an ice-themed deck. Despite all of this, Rio is extremely frightened of cats.
Rio is later revealed to be Merag, a member of the Seven Barian Emperors, who became determined to follow her brother no matter what happened to them and sided with the Barian World. She was absorbed by Vector after losing a Duel to him. She was later revived after Yuma and Astral's Duel.
Known as Merag, she was once the priestess as well as the princess of the United Lands of the Poseidon Ocean alongside her brother, Nasch. One day, the kingdom was attacked by Vector, who led a fleet of ships and several "Gorgonic Guardians" to lay waste to it and loot it. After Nacsh led the kingdoms forces to defeat the Gorgonic Guardians with mirror shields, Vector performed a blood sacrifice to the ocean god, Abyss, turning the battle back into his favor. Merag rode up to Abyss and was grabbed. She offered herself as a sacrifice, plunging into the ocean depths to revive another god. This action was due to Don Thousand hypnotizing her. As she owned a card called "Numbers 94: Crystal Zero, the Princess of Polar Ice", Thousand influenced her death to aide in his own revival.
An unknown number of years later, Don Thousand reincarnated both her and her brother as two of the Seven Barian Emperors, as their souls were each connected to a Legendary "Number" card, which had sealed Thousand's power. The two became the leaders of the Seven Barian Emperors. Vector had also been reborn as a Barian Emperor and often clashed with Nasch's leadership. After Nasch had annoyed him enough to score 100,000,000 points on his self-made point system, Vector took Merag hostage. When Nasch intervened, Vector used "Giant Sky Sword" to freeze Nasch's body and force both of them to jump off a cliff. Vector believed them dead, while Durbe believed they had vanished and assumed leadership of the group from them in the interim. The two were rescued by Abyss, who informed them that he would send them to Earth, where their souls would find new bodies to inhabit. The process would eliminate their memories though, until they reunited with Abyss again. Their souls were transferred to the bodies of Ryoga and Rio Kamishiro, who had recently died in a car accident.