Opal, also known as Opal the Dreamer, was a girl of the Plains that lived long before the time of Adin. She had the strange power to see visions of the future in her dreams. She is direct ancestor of Adin.
Opal was the daughter of two Plains farmers, Liza and Dodd. One year before Opal was born, Liza and Dodd found many small opals that made them rich and they named their daughter after the stones that had brought them such joy.
Because of the many dangers of the Plains, Opal was never left unguarded. Even when she was old enough to be allowed outside, she could never leave the farm and wore a veil to protect her from the bees and leather boots and gloves to protect her from the plains scorpions and fighting spiders.
Opal also had dreams, many of which depicted things that her parents later found had actually happened. Once, she dreamed of a man drowned in a river; her parents later found the same man, drowned in the river, just as she had dreamed. She later realised that her dreams were foretellings of the future; if she dreamed that something would happen, it would.
Visions of Adin
One night, she dreamed of the Shadow Lord's army invading Deltora and a man of iron and fire defending the land, with a belt filled with gems, one of which she recognised as the great Opal, talisman of the Plains people. She ran to the town of the Plains and warned the people of the Plains that one day, the Shadow Lord would attack and that they must listen to the man of iron and fire when he finally came. The people, however, took her for a crazy girl that had lost her wits and did not listen. Every day, she cried out her warning to people passing, until she finally left to find this man of iron and fire.
Eventually, she came to Del and met a blacksmith. This was not the man she was looking for; she realised that this man she had dreamt of had not yet been born. She married the blacksmith and so far away from her home, her dreams finally ceased. The man she had dreamt of was Adin, a direct descendant of hers, who would later become savior of the land.
Adin later mentioned that Opal would hold her children and her children's children in her arms and say that "they were not the ones." 
Battle for Deltora
During the Battle for Deltora, Adin realised that Opal's visions were coming true. After the battle, he deduced that the final vision of a ruined city was not foretelling Deltora's destruction, but an event in the far future.
Opal was a beautiful girl despite her strangeness. She had long, black hair. Most of her early life she had been hidden away from the world by her overprotective parents. They made her wear a veil when she was outside, so that the insects would not sting her. Because of this, not many people knew what she looked like.
In her youth, Opal was a silly girl who loved to play with and explore her surroundings. However, as she developed her powers, she became more dour and dutiful, to the point of running away from home in an attempt to warn the people of Hira. She also came to believe in fate and saw no reason to try and alter the outcome of her visions.
Opal had the unusual ability to see visions of the future in her dreams. The ability is implied to be connected to the opals her parents unearthed, and which Opal played with as a child.