Rui's step-sibling after their parents arranged a remarriage, though both have already known each other after their one-night stand, Rui personally wanting him to forget what happened. Their relation can be a mixture of sibling-like approach. However after Rui thought over everything of how she feels about Natsuo, she nevertheless accepted the fact she is in love with him.
Their relationship does well until Rui decides to be a chef after meeting her biological father. During the progress of her career to be a chef, she took his invitation to learn more in America for a year. At the same time, Natsuo had developed writer's block and didn't tell Rui, depending on other girls whom she didn't know. Feeling bad for being unable to help and figuring both of them were burdening each other, Rui decided to break up after much thought, turning their relationship back to brother and sister. Nevertheless, she still cares greatly for him.
After her horrible days in the work and losing her sense of taste, she finally understand how hard and scary was the life for Natsuo, when he needs her most and finally she regrets for her decision from break up with him falling in depression and guilt.
She got back together with Natsuo when he came to New York to help her through her rough time. Not long after, she learned she was pregnant with Natsuo's child.
She is Rui's elder sister. Deep within their relationship, they both envy their own personalities, with Hina wanting Rui's attitude and vice versa. They respect each other and hold an especially strong bond as sisters, and will be on the line if one of them felt at the bottom.
Rui was to first to know about Hina's relationship with Natsuo and initially let them be before she and Natsuo later became lovers. She even went far to not tell their current parents about their relationship.
Momo is Rui's high school best friend and fellow former club mate. The first encounter is a pleasant one but filled with contradictions due to Momo's influence at school as a "whore". Rui didn't acknowledge the rumors, deciding that it was up to her to decide if she wanted to spend time with her; she is, however, surprised with Momo's acknowledgement of having been in multiple relationships. Momo is currently unaware of Rui's feelings for Natsuo.
Miu is Rui's fellow clubmate and fellow second year. Rui first thought of her as a rival towards Natsuo, but Miu turned out to have a one-sided love for their club advisor and teacher, Reiji Kiriya.