Rivalun serves as the primary antagonist in the fictional anime series "Majokko Mirakurun," situated within the "YuruYuri" universe. She collaborates with her loyal subordinate Ganbo, pursuing their shared goal to worsen the world through schemes targeting the magical girl Mirakurun.

Operating under the secret civilian identity Raika, she adopts the appearance of an unassuming schoolgirl with mint green hair and yellow eyes. In this persona, Raika meticulously cultivates Mirakurun’s trust by projecting peacefulness and selflessness over time, concealing her villainous nature beneath a benevolent facade. This deception harbors an internal contradiction: despite her antagonistic role, Rivalun unknowingly nurtures unacknowledged positive feelings toward Mirakurun and cannot imagine existence without her rival. This duality layers complexity, juxtaposing her malicious actions with an unrecognized emotional connection.

Originally introduced in the "YuruYuri" manga, her role expanded significantly in anime adaptations. The anime incorporated original content centered on Kyōko Toshinō’s doujinshi creations based on "Majokko Mirakurun," depicting two Comiket trips where Kyōko and others sold these fan works. This integration solidified Rivalun and the "Majokko Mirakurun" universe within the main storyline’s events. Her documented involvement remains confined to this narrative and the Amusement Club’s related doujinshi activities.