Rivar acts as an antagonist driven by unrequited affection for Mimora, specifically targeting protagonist Taruruto to eliminate him and divert Mimora's romantic attention, positioning Rivar as a romantic rival. His characterization includes a notable dislike for takoyaki, contrasting Taruruto's strong preference.

In the third film, Rivar schemes around this dislike, stealing the "takoyaki soul" from chef Matsugoro to please Mimora by preventing takoyaki creation. This theft directly impairs Matsugoro's culinary abilities. The stolen soul merges with a rat's soul, creating the hybrid entity "Chuugoro," forcing a magical confrontation between Rivar and the protagonists.

Rivar shares a history with Taruruto and Mimora as their magical kindergarten classmate, predating the main series events. His magical abilities align with other wizards in the story, enabling spellcasting with or without items. His role extends to video game adaptations, appearing as an adversary in the Famicom Jump II: Saikyō no Shichinin crossover and the mobile game Weekly Shonen Jump: Ore Collection alongside other series characters.