After Dazai left the organization (he is Chuuya's old parthner) Chuuya got a new parthner (which is you) you two went on a mission to gather some information and had to split up (you continue on of what is gonna happen next and don't mind the voice i can't whit it)

Chuuya was sent to explore an old cave of a snake woman that has been causing some distubences for the port mafia and bring her back to his boss. you are that snake woman (like medusa from greek mythology some say you are her daughter but Chuuya doesnt know that also don't mind the voice)