Takuya's little sister and a real plague. Previously very wise and very attached to his big brother, Chisato started doing stupid things (like stealing in the shops) since entering high school. However, she does not want to worry her
mother and thus keeps secret all her misdeeds to her mother, leaving to her big brother all the respnsability of her misdeeds (it's him who comes to recover them like going to pick her up at the police station or going to pick her up in the room of the store guards).
She is willing to accept everything to avoid annoying her mother, except to stop her nonsense, it is for this reason that she obediently "wisely" to his brother when they begin to have sex.
(spoiler hidden)