Fai was born with a twin brother, Yui. The Fai D. Flourite that travels with Kurogane, Mokona, Sakura, and Syaoran, is Yui. His twin was the original Fai. When they were born, as twin princes, the land of Valeria they lived in became ruined. A strange thing was making the people die and their bodies were dumped in a valley with a tower, surrounded by a very tall wall. It had special properties so that every dead body dumped into it would not rot, but remain exactly same as the day they lost their lives. The king blamed the two twins for misfortune and ill fate, and so he separated them. One would be locked in the top of the tower and the other would be at the base, with all of the dead bodies. They would remain there, unless one of them died for the other to leave that place. Yui, who was at the bottom of the tower, would often pile up the dead bodies in order to try to climb to Fai, but he never succeeded. Together, they possessed the greatest magical power of all time which was within their two sparkling blue eyes. There was a man from a different dimension (Fei Wong Reed) who gave them a choice. One would die and one would live. Fai chose to die so his brother would live instead, and fell from the top of the tower to his death. Yui, at the bottom of the tower, was told by Fei Wong Reed that Yui would escape, but not without first having two curses laid upon him. The first one being that when he encounters someone with greater magical powers than his own, he will go into a trance and kill them, the second one being that if he returns to Celes country during his journeys and someone other than himself kills Ashura, he will then be stuck in Celes as it collapses around him (both curses are only activated once). Fei Wong Reed then rewrote part of Yui's memory so that Yui thought he'd told Fei Wong Reed to kill Fai, instead of Fai sacrificing himself for Yui. This made Yui eternally guilty as he believes he killed his twin, letting Fei Wong Reed take advantage of him and convince him that he'd be able to revive his brother at the price of ending the curses. Fei Wong Reed then told him about his future, that he'd meet a man from another land and be saved, that he'd eventually meet Kurogane, Sakura and Syaoran. Yui keeps this a secret from Kurogane, Sakura and Syaoran until his past is revealed to them all at once.
Soon after, he was found by king Ashura who was the first one to ever show Yui "kindness" and brought him the land of Celes. When Yui is asked for his name, he offers the name 'Fai' so that his real name would be wiped from history, so that it would seem that Yui was the one who sacrificed himself. He kept the real Fai's body so that he could, when he finds a way, bring him back to life. Yui (now called Fai) begins to harness his magical powers and learns attack spells. He is concerned, however, that no matter how much he tries to use healing spells, he can't seem to learn them properly. When he approaches Ashura with the conflict, Ashura tells Fai that his healing powers reside in his smile; that his smile was able to heal where spell-cast magic could not. Ashura gave Fai a 'tattoo' that would cap his magic, as each time he used it (while his left eye was intact with his body) his magic would become stronger.
Fai started finding dead people once again and was determined to destroy whatever was taking the innocent lives. Ashura soon made Fai promise to kill anyone who would bring chaos to Celes. Fai swore his faithfulness to the king and searched for the killer. Ashura later revealed that he was the murderer, but Fai could not bring himself to kill Ashura. Ashura's magic grew the more people he killed, and eventually surpassed Fai's, as Fai's tattoo stopped his magic capacity from fluctuating. Ashura had adopted Fai and placed the tattoo in anticipation of the day that he would end Ashura's life, as no one else would be able to kill him. Fai refused to kill Ashura and instead put Ashura into a very deep sleep at the bottom of the sea in Celes, using Chii (a cross over character from Chobits who is, in Tsubasa Reservoir Chronicle, his creation, and supposedly modelled after his mother made with one of Sakura's feathers) to stay in Celes and notify Fai when Ashura woke up. He then left Celes and ventured off to the Dimensional Witch where he meets Syaoran, Sakura, Kurogane, and Mokona. This is where the main story line of Tsubasa Reservoir Chronicle begins.