Otoya is a supporting character in MAO who appears as the young assistant of the onmyoji Mao. He has the outward appearance of a small boy, but he is not human. Otoya is a shikigami, a kind of spirit familiar, created when Mao infused his spiritual power into a small wooden doll. That doll, which resembles his childlike form, serves as the vessel for his spirit, and this truth about his nature is revealed over the course of the story.

In everyday life, Otoya works as Mao's loyal assistant. He carries Mao's bag of tools, fetches whatever the doctor needs, and generally handles practical tasks so that Mao can focus on his work. He speaks politely and refers to himself as "this one." Despite looking like a child, he has a quiet, mature, and composed personality that is unusual for his apparent age, and he shows no signs of the mischief or playfulness typical of young boys. He is usually calm and nearly expressionless, and he proves to be an earnest and dependable companion.

Otoya is motivated by a strong sense of duty to Mao. One of his most important responsibilities is minding Mao's medicine jar, the only thing that sustains Mao when he is severely injured. When that jar is destroyed, Otoya takes on the painstaking work of assembling a new one and finding poisonous creatures to place inside it. Because he is not a living being of flesh and blood, he can survive damage that would be fatal to a human; even if his body is sliced in half, Mao can repair him.

Within the story, Otoya also serves as a patient guide to Nanoka, the girl who enters the Taisho era from the modern world. He calmly explains unfamiliar things to her and, like Mao, is puzzled by her modern behavior and knowledge. His role is mostly supportive rather than central, but it is significant: his loyalty, reliability, and willingness to tend to crucial behind-the-scenes duties make him an essential part of Mao's daily life and operations.