Aya Kōda is a civilian who assisted Doppo Kunikida in a bomb-related case in Yokohama. Aya is a spirited girl who calls herself a warrior of justice and practices martial arts every day. She is stubborn, refusing to take no for an answer, and makes fun of Kunikida often. She seems to value helping others and is willing to sacrifice herself for the sake of others. Moreover, Aya is quite bright, able to deduce whatever is inside a coffin.