Doua is always called a child, even by Rin, indicating Doua is likely 13-15 years old.
She is very small for her age, and very fast. She carries a heavy foreign sword with ease and often wears a checkered hat in battle, as it is plated in iron on the inside.
Doua is Japanese and was left at the door of an Ainu household as an infant, and therefore raised in Ainu culture until about the age of twelve. One day she came across Isaku and his father and their cabin, and continually showed up to hang around. She and Isaku got to know each other, and one day she arrived saying she wanted to leave and travel, and hoped Isaku would go with her. She explained she passed out when her family started giving her the tattoo on her mouth, and later conceded that it was not so much the pain as being marked for life by a tattoo that is not her own which made her upset.
Isaku and his father come to terms with each other, and Isaku leaves with Doua. Doua asks him to help give her her adult name, and he chooses "Doua". Doua's childhood name was "Kuichiru".