A lack of sources makes it difficult to discover the personhood of premodern Japanese royal women—how they viewed themselves or wanted to be viewed by others. Hachijō-in (1137–1211), a royal heiress, strove to protect her lineage, which had begun to weaken in the political and military turmoil of the late twelfth century. Through a close analysis of her prayer of dedication (kōmon), this article explores how she reflected upon herself after undergoing a time of great difficulty as an unmarried royal daughter. In contrast to the historical overviews that typically emphasise the roles of medieval male royals and warriors, this study demonstrates that royal daughters also played significant religio-political roles. They developed a strong sense of responsibility for supporting royal authority and commemorating deceased family members. Hachijō-in’s prayer reveals her objectives, feelings and values as a woman through a deliberate construction of herself in front of the Great Bodhisattva Hachiman of Iwashimizu Shrine.