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-Background of my Character- I cosplay, spoiler alert.
When Fai's past is finally revealed, it's known that his real name is Yui. It is shown that "Yui" was born in Valeria Country, with a twin named Fai. Though the birth of the second prince's son was highly anticipated, the birth of twins 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 them unhappy was the only way to break the "curse", and since each refused to kill the 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 Yui 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. Yui tried endlessly to climb up towards Fai, so they could escape, but he did not succeed. Eventually, the king became insane, and he executed everyone in the country, except for Yui and Fai. He then jumped into the valley himself and committed suicide in front of a horrified Yui, 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 Yui, resulting in Fai being thrown from the tower to his death. Fei Wong then tampered with Yui's memory to make him think that Yui had chosen to save himself at the cost of his brother's life, allowing Fei Wong to use his guilt in order to turn him into his servant. In order to bring his twin back to life, Yui allowed Fei Wong to bestow two curses upon him, one of which was removed from Yui's memory. Fei Wong then told Yui that someone would free him from the valley, and that he would have to go on a journey, protecting "a princess" (Sakura), and killing any obstacles along the way. Fei Wong then tells Yui that when he finds a magical feather, he should put it into Fai's body to prevent it from rotting, and that he should begin his journey when he finds a second feather, which he should give to Sakura.
In Shiritsu Horitsuba Gakuen.
Yuui (Home Economics), calmer.