AppearanceEditDue to having a child-like appearance, Seiji initially mistook her for a grade school student. She has long dark hair, round dark eyes and light skin.
Riko is shown to be quite bossy and sometimes outright rude, especially towards Seiji, who she considers herself to be an older sister to. This is mostly also due to Seiji being a pushover.
However, she also possesses a more childish side, in which she admits to Seiji that she dislikes being left alone. Also, she has admitted to also feel very sorry for her brother, who troubles Yuki often to make her want to help him, resulting in Riko informing him of Yuki's whereabouts. She later regrets this.
Riko, along with her brother, were friends with Yuki during their childhood. The 3 would often play together. It is unknown what grade school she attended when Yuki and her brother were tricked into becoming intimate.
Riko, a 3rd year of Junior High, makes her debut when she runs into Seiji, trying to retrieve her photographs from the same shop. She displays rudeness towards him before running off. The two later encounter when Riko's boss requests to see him, since Riko had accidently picked up the wrong envolop and brought back Seiji's photographs, which turn out to be very good.
Although being very hostile, she later warms up to Seiji.
Sometime later, she encounters Yuki, who is with Seiji. The two recognize each other immediately, and Yuki runs off in desperation. Seiji later questions what she knows about Yuki's past, and she reveals her connection to Yuki's ex-boyfriend, Tooru, who is her brother.
As expected, and out of pure concern, Riko informs her brother of Yuki's location, and upon realizing what bad impact that has had on Yuki later on, she admits to the group who try to find a missing Yuki that she was sorry and didn't think this would happen.
With no leads on the whereabouts of the pair, Seiji demands the know of a place that they likely went to, and Riko informs him of an abandoned building, which is shown to be where the two are.