Yuui's twin brother appearing in his flashbacks and supposedly held in Celes Country.
Show SpoilerIn Valeria Country, though the birth of the second prince's son was highly anticipated, the birth of twins (Yuui and Fai) was considered to be a bad omen. Shortly after their birth, the second prince died, and their mother committed suicide out of guilt for giving birth to them. In addition, the country suffered many misfortunes, and the twins, when combined, bore powerful magic that rivaled that of the current king.
The king decided that making Yuui and Fai unhappy was the only way to break the "curse", and since both twins refused to kill each other, they were locked up separately in a place where magic is ineffective and time does not flow: a tower residing in a deep valley; Fai was imprisoned atop the tower, and Yuui was confined at the base of the tower. Both were condemned to stay there "until the world's destruction."
Despite their unhappiness, the number of bodies dumped into the base of the tower began to grow. Yuui tried endlessly to climb up towards Fai, so they could escape, but he did not succeed. Eventually, the King became insane, and he executed every person in the country, Beside's Yuui and Fai. He then jumped into the valley himself and committing suicide in front of a horrified Yuui, reminding him that their birth was the reason for the country's misfortune, and that they would now "pay for that sin" by being the only people alive in Valeria.
Eventually, Fei Wong Reed appeared from another world, and offered to free one of them. Fai chose to save Yuui, resulting in Fai being thrown from the tower to his death. Fei Wong then convinced Yuui that he was the cause of Fai's death and that Fai might be revived if Yuui agrees on becoming Fei Wong Reed's pawn.
After meeting Ashura, Yuui took up the name of his dead brother as his own. The body of real Fai was kept underwater in Celes castle by Yuui. It is also revealed that Yuui is in posession of two of Sakura's feathers. One feather Yuui kept intill he met Sakura so that she woulden't die as soon as she lost her memories. The other feather Yuui used to create his assisstant, Chii. Chii was meant to be a compainion for Fai seince Yuui new he woulden't alway's be in Valeria, And also so Fai's body would not continue decaying. Chii was also created to look like Yuui and Fai's mother.
Ashura placed a flourite with Fai the same time that he gave Yuui the name of flourite and it was later reviled that it held one of Sakura's feathers inside it.
In Horitsuba Gakuen, he is the twin brother of Fai (in Horitsuba Gakuen) and is a chef from overseas.