Prince Mirmo rules the Fairy World (Muglox world), standing just 10 cm tall. Fleeing his homeland to escape an arranged marriage to Princess Rima—whom he saw only as a friend—led to his summoning by human teenager Katie Minami via a magical cocoa mug, forging their partnership. As royalty, he wields the power to temporarily transform humans into fairies for 24 hours.

His personality mixes playful charm with selfishness and laziness, frequently chasing chocolate treats like "KumoCho" (Cloud Chocolate) over duties. Despite these flaws, fierce loyalty and courage surface during critical threats like the Warumo Gang or Darkman. He fights using magical maracas and a comedic "Butt Fist" attack.

His relationship with Rima evolves; while consistently resisting her romantic advances and proposals, he occasionally reveals hidden concern. Partnered with Katie, he aids her romantic pursuit of Dylan Yuki, though his methods often trigger chaotic mishaps. In the anime's conclusion, after securing Katie's wish with Dylan, Mirmo faces expulsion from the human world. Erasing Katie's memories to spare her pain, he fails to return to the Fairy World, suffering severe punishment: transformation into a rabbit and complete memory loss. Katie's persistent care ultimately restores his memories and form.

Manga continuity sees his character development culminate in accepting his feelings for Rima, leading to marriage—contrasting the anime's unresolved romance. Rivalries, like that with ninja fairy Yatch, swing between comedic antagonism and reluctant cooperation during missions to protect human partners or save both worlds.

Mirmo exhibits leadership in major arcs, reversing the Fairy World's time freeze and aiding the robotic octopus Tako in gathering emotion-based crystals. His younger brother Murumo acts as both annoyance and ally, showcasing Mirmo's role as an exasperated yet protective sibling.