Sakae Jinnouchi is the 89 year old matriarch of the Jinnouchi family.
Sakae is depicted wearing red glasses and traditional Japanese clothing. She has short, gray hair and dark brown eyes. When she smiles, Sakae has two teeth in the top row that stick out.
Sakae is first mentioned when her granddaughter, Natsuki Shinohara, says that it is her grandmother's birthday and asks if Kenji Koiso wants to come with her. She is duped into believing that Kenji is actually Natsuki's boyfriend, even though he isn't. Later, after Kenji receives the enigmatic message and OZ stops working, she encourages important people to keep doing whatever they can until OZ starts up again.
Later on, Natsuki's uncle, Wabisuke Jinnouchi, appears and has an argument with Sakae about Love Machine. Things get really heated when Sakae picks up a sword and almost attacks Wabisuke, who promptly drives away in his car. Afterwards, Sakae and Kenji play a game of koi-koi while having a talk. Sakae tells Kenji that even though he is not Natsuki's real boyfriend, he should take care of her.
A little later, Sakae Jinnouchi dies.
Her death inspires Kazuma Ikezawa, Kenji, and a few others to catch Love Machine because Sakae wouldn't have died if OZ was still functional.