Seia Onoue holds the rank of Colonel in the Japan Ground Self-Defense Force, serving as a high-ranking officer within its military defense forces. Her professional duties involve the Brave Police initiative. Her younger brother, Makoto Onoue, serves in the same defense forces under her command.

She possesses shoulder-length maroon hair and blue eyes. Her typical presentation includes wearing lipstick and earrings, reflecting an adult, professional demeanor.

Her character arc is significantly defined by her relationship with McCrane (BP-301), a sentient robot member of the Build Team. This bond forms after McCrane saves Makoto Onoue's life. Their connection deepens through interactions, characterized by mutual respect and a strong emotional attachment, with narrative implications of developing romantic feelings towards the robotic McCrane. This evolving relationship is central to her story.

Her personality reflects her military background, typically maintaining a composed and professional attitude. However, her interactions with McCrane reveal emotional depth and loyalty, particularly shown through her concern during periods when McCrane experiences violent impulses or external manipulation, which impacts their dynamic.