Riki Muroi, grandson of scientist Goken Muroi and identical twin to Gaku Muroi, encounters Persia Hayami with his family in Africa, subsequently bringing her to Japan where she resides with adoptive parents in Minato-machi. He develops a protective, sibling-like care for Persia.

Bold, athletic, and outspoken, Riki contrasts sharply with his more sensitive brother Gaku, embodying the energetic "red oni" to Gaku's analytical "blue oni." His physically active nature is demonstrated through participation in school sports, particularly American football. Riki pursues a romantic relationship with Sayo Mitomo, Persia's best friend, whose grounded personality aligns more with his conventional demeanor compared to Persia's wild background.

Initially, both twins admire Persia's magical alter ego without recognizing her true identity. Riki does not return Persia's childhood crush, maintaining their sibling bond. When Persia's romantic feelings shift towards Gaku, Riki supports their relationship, acting as a supportive brother to both. Within the established narrative, his primary role involves assisting Persia during her missions to collect love energy for Lovely Dream, often alongside Gaku and their grandfather Goken, who serves as their parental figure in Japan.

Riki shares a close bond with his twin brother, frequently appearing with him unless occupied by separate school activities. His character role and backstory remain consistent across all documented media.