Kagami is love interest of Utsu no Miko, she was a childhood friend of his and her feelings for him grew into love. On their journey into the heavens, she is kidnapped by the god Taishaku-ten, whom becomes enchanted with her and Utsu no Miko struggles throughout the quest to rescue her.

In the chaos of the Jinshin-no-Ran civil war of 762, a child with a small horn in his forehead was born. The child's mother condemned him as an oni and cast him away. An elderly sh&#363;genja woman claimed the child and named him Utsunomiko, or 'Divine Child of the Heavens', telling Miko that his horn symbolizes the union of heaven and earth. Miko matured in the wilderness learning the ways of Shugend&#333;, and soon started venturing into villages out of curiosity. He found that the common people of the villages live in poverty and suffering, and began using his spiritual powers to help them. But his anger at the self-serving rulers and their petty power-struggles grew until he came into open conflict with the Imperial Court, setting Miko down a long path as a champion of the oppressed. 
(Source: Wikipedia)

Delving into the humans' spiritual journey to the Celestial Realm of Buddha, UTSU NO MIKO follows five friends on their quest to bring peace to a corrupt and chaotic Earth. 
Born with a horn, Utsu no Miko found refuge on Mt. Kongo where he studies to become a God. But when the ruling imperial government seems to bring only devastation to the people, Miko and his friends venture on a 33-year quest to reach the Celestial Realm and meet the almighty Father of the North Star. Yet, the way is filled with danger, for the five humans intrude upon the world of immortals where the gods reign supreme. And, in order to reach their goal, Miko must first challenge them to battle. 
However, they soon realize that the gods too are not without flaws and earthly desires. And for Miko, the hope of rescuing his love (Kagami) leads to the realization that the world cannot exist without the balance of gods' tumultuous soul, and the humans' will to survive and to love. For, as one life ceases, another takes its place. And only when there is true peace can people finally see the face of God. 
(Source: AniDB)