Hideo Shiina is the father of the main protagonist, Tsubasa Shiina. He is a dedicated parent who makes a significant life change by moving with his young daughter from Tokyo to the rural landscape of Hokkaido. This relocation is motivated by his personal dream of pursuing a new career as a baker, and he begins working at the Ibaragi ranch to achieve this goal.

In the story, Hideo plays the role of a supportive and hardworking single father, running their household and caring for Tsubasa after the death of his wife. He is unaware of the secret alien conflicts his daughter becomes involved in. A key aspect of his character and his role in the premise is that his memories are altered by the alien D.D.. This memory manipulation is a necessary measure to explain the sudden appearance of Hikaru, a duplicate of Tsubasa created from a living bio-metal. As a result of this manipulation, Hideo fully accepts Hikaru as his second, long-lost twin daughter, never questioning her sudden presence in his home and family life. He treats both Tsubasa and Hikaru equally with fatherly care, unknowingly providing a stable and loving home environment for Hikaru as well. His primary role remains that of a caring, if oblivious, parent whose dream of baking provides the backdrop for the family's new life in Hokkaido.