Riku Harada is the older of the twin sisters. Unlike her younger sister Risa, who prefers fashion and obsessing over relationships, Riku is more into athletics (lacrosse being her sport of choice) and academics and can be considered to be somewhat of a tomboy. She acts "less girly" compared to her sister Risa and is more serious, independent and down-to-earth. In terms of physical features, her hair is much shorter (about shoulder length) than her sister's and she displays more stamina and fitness (Risa is chauffeured to the train station then rides Riku's bike to school compared to Riku riding her bike to meet her sister then jogging the rest of the way). She is also kind and caring and very outspoken and selfless, only wanting the best for everyone and willing to help those in need.

She holds an intense dislike for Dark Mousy ever since their first encounter in which he had spotted her after she was done taking a shower then stealing her first kiss. In the anime, he landed on her balcony as she stood outside. Upon seeing him she was startled and in his attempts to quiet her down as to not attract attention he kissed her. In the manga, the same thing happened when he hid in her room escaping from the police and she woke up. The fact that Dark stole her first kiss added to her extreme dislike of him.