Kotone is the elder sister of Tomoya and a lawyer. She cares deeply for her brother and regrets the fact that she and her parents leave him alone at home so much.
She has short blonde hair and wears glasses. Besides sharing a hair colour, she and her younger brother Tomoya are quite different—Kotone is a lot shorter than Tomoya, who maintains a happy expression most of the time while she becomes openly stressed and flustered.
Kotone has a stalwart personality that drives her to work hard for people in need. In this regard she takes after her parents, who also have workaholic tendencies. Should she fail to help others, she becomes incredibly downcast.
Her first appearance is on Tomoya's birthday when she apologises for leaving him alone on his birthday for work, regretting that their parents are already away and that even she cannot celebrate it with him. It is revealed that she had tried to bake him a cake by the end of the day, but each attempt went poorly, though he appreciates the effort.