Megumi Ooumi is a central human character in the battle to decide the king of the mamodo world, serving as the book owner and partner to the mamodo Tia. She is introduced as a sixteen-year-old who lives a double life as a highly famous and successful J-pop idol. Her career is the result of a long-held dream to bring happiness to people through her songs, and she has achieved stardom while still being described as a regular girl at heart. Physically, Megumi is a tall, slender young woman with long brown hair and large brown eyes. Her casual clothing often features green and orange colors, while her stage persona involves elaborate, princess-like dresses. Her name, written with characters meaning "large ocean" and "blessing," reflects her generous nature.

At her core, Megumi is defined by a remarkably benevolent, kind, and charismatic personality. She is rarely seen angry and consistently strives to help others without hesitation, often offering her perspective to broaden the minds of her companions. Megumi views her mamodo partner Tia not as a tool for battle but as a younger sister, patiently calming the younger girl's frequent fits of rage and explaining the complexities of the human world to her. Despite her fame, Megumi longs for a normal life, a desire that stems from the exhausting and overburdening schedule she maintains as a pop star, which sometimes leads to sleeplessness. She is also noted to be skilled in judo. A subtle but recurring aspect of her personality is a romantic interest in Kiyomaro Takamine, Zatch's partner, which is shown through her blushing and occasional jealousy, even though Kiyo himself does not seem to acknowledge these feelings.

Megumi's entry into the mamodo battle was an act of compassion. She found the mamodo child Tia drowning in the ocean and rescued her. After Tia recovered and explained the nature of the battle, Megumi chose to become her partner despite Tia's initial wishes to protect her from the danger. Their early time together was fraught with fear, as they were relentlessly pursued by Maruss, a former friend of Tia's who had betrayed her. This situation left Megumi feeling so unsafe that she considered canceling her concerts, an idea Tia firmly rejected, as she wanted Megumi to continue pursuing her dream.

Megumi's primary motivation is her devotion to Tia and her desire to protect the people she cares about. Her initial hope to form alliances with other mamodo teams was forbidden by the traumatized Tia, but after being rescued and befriended by Zatch and Kiyomaro, Megumi came to see them as trusted friends and swore to one day return their kindness. This relationship profoundly shapes her journey, as she supports Tia in learning to trust others again and ultimately shares Tia's goal of helping a kind king ascend to the throne to end the suffering caused by the battle.

Within the story, Megumi and Tia serve as one of the first major allies to the protagonists and are considered key supporting characters. Their role is defined by their unique abilities; while Tia specializes in defensive and healing magic, Megumi is the courageous human partner who reads the spells from the book, enabling their use. Together, they form a defensive powerhouse, providing crucial support in larger battles. Their fighting style reflects Megumi's protective feelings, which directly power Tia's shields. Among their notable spells are Seoshi, a transparent protective barrier; Ma Seshield, a powerful disk-shaped floating shield; Saifogeo, a floating sword that heals injuries and restores heart energy; and the conditional attack spell Chajiru Saifodon, whose devastating power is fueled by Tia's anger over bitter experiences, reflected in a mirror on the spell's goddess-like form.

Megumi's key relationships are central to her character. Her bond with Tia is the most significant; their partnership is built on mutual care, with Megumi providing the emotional stability and trust Tia desperately needed after Maruss's betrayal. Her relationship with Kiyomaro is friendly and tinged with unspoken romantic affection, while she regards Zatch as a good-hearted mamodo and a worthy future king. Throughout the series, Megumi develops from a young woman living in fear, constantly on the run, into a more resolute and proactive participant in the battle. Her courage is reinforced by the example of Zatch and Kiyo, and she becomes instrumental in helping Tia heal her emotional wounds. After the battle concludes and Tia is sent back to the mamodo world, Megumi continues her life as a pop idol, though she is later shown visiting Zatch and Kiyo, visibly missing her departed partner.