Yui is Akira's older twin brother.
He is the former user of the nil calculus. In the snow village of mount Iwanki, Yui and his twin brother, Akira, were born. It has been said that a male Yuki-onna in their village is born once every hundred years. When growing up, the two would go near the barrier that protects the village but, kept getting caught. In a magazine Akira found on the mountain, he and his brother found about the Inugami detective agency and Tokyo.
When the women of the village are deciding which of the twins should be chief, Yui volunteered to be it as he didn't want Akira to suffer. As chief, he would have sex with the women to make sure the village survived. He asked the women of the village to let them out of the barrier and promised them he would impregnate all of them if they accepted his request. They agreed and Yui talked to his brother as they are going near to the barrier. He told him that he was leaving the village too but, as they are getting close, Yui left him. Before he was completely out of Akira's line of sight, he told him to go to Tokyo, find the Kemono office, and they will see each other again.
Later, Yui would go to the place where it possesses the Nil Calculus that created the barrier for the snow village to escape the women. He put the Calculus in him, wiping the village in the process. As the calculus possessed him, he only remembers what was important to him, his brother. Leaving in search of him and he was later reunited with him.
