Rio is the trainer assistant in the ranching sim video game, "Monster Rancher 4". Though initially a very shy and introverted, even fearful girl, Rio eventually came out of her shell to aid Phayne in his quest to become a successful breeder. She has the power to talk to monsters, a trait which many feared and hated Rio in turn. However, after some encouragement from Phayne and Mockruji, she learned to embrace her ability
In an ancient time known as the Dark Age, Rio was the leader of a fierce battle between humans and good monsters versus evil monsters, and their leader, the Dark Lord, Xevion (alternatively spelled Zevion). Despite many casualties on the side of the humans, the Dark Lord fell, bringing an end to the Dark Age. However, Suzaku, Lord of a legendary breed of monsters called the Phoenixes, fell in the battle. The remains of his body were sealed into four saucer stones, called the Gaia Stone by Rio and Phayne.
Rio fell into a state of deep unconsciousness in the battle, as well. The humans who fought in the battle decided to seal her in a saucer stone, with hopes of her being reborn and living as a normal child in a future age.
Millenia later, Cesare, the priest of a Togle shrine, took a saucer stone to the shrine, in hopes of regenerating a monster. Out of the stone came a baby, Rio. Cesare decided to raise her as his own daughter, lying to her about her true identity.
Years later, when Phayne came to the ports in Togle and met up with Cesare, Cesare decided to give the recently-expelled Phayne a second chance at monster breeding. Cesare appointed Rio to show Phayne to his ranch, and Rio obeyed. After a brief social encounter, Rio took her awkward leave, leaving Yuri to assist Phayne on the ranch.