Mie Daiwa is the adoptive mother of the series' protagonist, Yamato Daiwa. She discovered and adopted five-year-old Yamato on July 6th, rescuing him from being raised solely by wild cats; this date became treated as his birthday. She operates her restaurant, initially called "Mie's Café" and later renamed "Mie's Cat Cafe." Mie managed the cafe primarily alone but eventually enlisted help from Liena, Armada, and members of Yamato's friend group for daily operations and chores.

Her management style emphasized discipline, especially monitoring Armada to ensure he didn't shirk duties, believing this enforced responsibility. This strictness sometimes made others apprehensive. Mie combines warmth and concern, particularly towards Yamato and his friends, as a loving yet occasionally anxious parent attentive to his development and connection to his B-Daman, Cobalt Blade. She shows compassion to former antagonists, expressing sympathy for Li after his defeat when others, including his brother, did not. However, her trusting nature led to naivety; despite knowing Li was associated with the villain Ababa, she mistakenly assumed he was just another friend of Yamato's during an encounter. This misjudgment nearly resulted in her kidnapping, mirroring Liena's earlier abduction.

Physically, Mie has dark brown hair and light blue eyes. She is often depicted wearing practical attire: a red smock and a yellow bandanna. Both her ears are pierced, but she typically wears no additional accessories beyond these. Her background centers on her role as Yamato's caregiver and cafe proprietor, with documented changes limited to the cafe's name shift and her ongoing efforts to manage it with assistance. Her primary narrative function is providing Yamato a stable home while running her business.