Setsu appears in episode 14, "Inside the Cage." Mysteriously, she was born as a bamboo shoot, but grew as a normal human - outside of surviving only on the water of the white bamboo, that is.

She is what they call an oni-ko, the daughter of a human and a mushi. She is like a normal human except needs the water of the white bamboo mushi, the mataradak'e, to survive. She is the daughter of the mataradak'e and a human mother