Himiko Day is the matriarch of the Day family in B-Daman Fireblast. She is the wife of Greg and the mother of Kamon, Aona, and Ryoma. Her professional background lies in archaeology, where she is established as a colleague of Gennosuke Shirogane, the father of another key character, Samuru.

In terms of personality, Himiko is generally an outgoing and caring individual, particularly toward her children. She is also a loving wife to her husband, Greg. However, this warm nature is balanced by a notable hot temper, which emerges when she becomes upset, a trait she shares with other members of her family. Her personality also includes playful quirks, such as affectionately calling the B-Daman Drive Garuburn by the nickname Garu-chan, much to the B-Damans embarrassment. In a notable contrast to her daughter Aona, who is a skilled cook, Himiko is shown to be terrible at cooking.

Her role in the story is primarily tied to her family and her archaeological work. She appears as part of an expedition alongside Samurus father, searching for a legendary B-Daman known as the Phoenix, which is tied to Crestlands history and involves the Four Symbols. Himikos return also coincides with Kamon regaining his long-forgotten memories, allowing for a reunion with his mother.

Her key relationships are central to her character. She is a devoted and loving wife to Greg. As a mother, she is caring and affectionate towards Kamon, Aona, and Ryoma, sometimes to the point of embarrassing them. Her professional relationship with Gennosuke Shirogane as fellow archaeologists drives her involvement in the expeditions that form part of the plot. Her notable abilities lie in her field of expertise, as she is a knowledgeable archaeologist with deep familiarity regarding the legends and history of Crestland, such as the tale of the Phoenix and the Kirin.