Mao, also known as the Demon Lord, is a central character in the anime Endro~! who begins as the primary antagonist but gradually becomes a key supporting figure. Originally the 999th Demon Lord, Mao confronted the hero Yusha and her party in a climactic battle. When Yusha attempted a sealing spell known as Hell Shield, the magic malfunctioned and sent Mao three years into the past, shrinking her to the size of a small girl in the process. Stripped of her full power and stranded in a timeline before Yusha became a hero, Mao decided to enroll as a teacher at Yusha's Adventure School. Her initial motivation was to sabotage Yusha's training and have her expelled from the school, thereby preventing the future hero from ever rising to oppose the Demon Lord. Despite her scheming intentions, Mao's naturally earnest and kindhearted nature caused her plans to repeatedly backfire. She took her teaching duties seriously and imparted high-quality lessons on monster weaknesses, medicinal herbs, and adventure fundamentals, unintentionally helping the very students she meant to hinder. Over time, Mao grew fond of Yusha and her friends, and after witnessing their earnestness and camaraderie, she abandoned her original plan and decided to forsake the path of the Demon Lord entirely, finding fulfillment in her role as an educator.

Mao possesses a sharp intellect and extensive knowledge obtained from rigorous training since birth, making her an effective instructor despite her small stature. She has the ability to transform into a giant, powerful Demon Lord form when she fights seriously, though she rarely uses this power in daily life. Her appearance is childlike, with short maroon hair, green eyes, elf-like ears, small fangs, and wings, and she is often mistaken for a young child. Among her notable experiences is contracting Demon King Fever, a disease that even demon lords can catch, during which she was nursed back to health by Yusha and her party, deepening her bond with them.

Mao's key relationships include her connection with the knight who originally gave her the teaching job at the school, and most importantly with Yusha, the hero she was destined to oppose. Unlike a typical adversarial dynamic, Mao becomes a supportive mentor figure to Yusha and her friends, and the students remain unaware of her true identity as the Demon Lord for most of the series. Mao also interacts with Princess Rona, who initially mistakes her for a child, and with her own assistant, a stone golem created to serve the Demon Lord. Over the course of the story, Mao develops from a lonely, duty-bound Demon Lord into someone who values friendship and a peaceful life, reflecting a journey of personal growth and self-discovery.