Yori Yuuki, the 15-year-old twin brother of Iku Yuuki, possesses high intelligence and a reserved, occasionally cold demeanor that conceals profound emotional vulnerability centered on his sister. His childhood affection for Iku evolves into romantic love, compelling him to initiate a secret relationship despite agonizing conflict over the incest taboo. In an attempt to escape these feelings, he briefly dates Iku's friend Tomoka Kusunoki and arranges a transfer to a distant high school, yet fails to weaken his attachment.

A genetic test later reveals Yori is not the biological son of Iku's father, Shunpei Yuuki. Instead, he results from an affair between his mother Saki Yuuki and Dr. Yuugo Mori, who conceived Yori during a liaison on Saki's wedding day. Iku remains Shunpei's biological daughter, confirming the twins as half-siblings. Upon this discovery, Yori and Iku confess their relationship to their parents. Facing familial and societal condemnation, they flee home, covertly living as a couple with aid from Yori's friend Haruka Yano.

Yori's inner turmoil escalates during this period. He wrestles with guilt over societal fallout affecting Iku and their fractured family. Convinced separation offers their only path to healing, he unilaterally ends the relationship. Without explanation, he abandons Iku in Haruka's care and vanishes to an undisclosed location, intending time and distance to dissolve their bond.

A decade later, Yori reunites with Iku in England. Despite his efforts to sever their connection, he confirms his enduring love remains unchanged.