Emma Daiōji is a second-year student at Y-Academy, an elite boarding school admitting only students with top-class abilities. She is the daughter of the academy's principal, Kinya Daiōji, who deeply cherishes her. Emma possesses fair skin, golden-yellow eyes, and long light blonde hair styled with bangs and three curved hair spikes on each side of the bottom length. Her uniform consists of a black gakuran shirt featuring a white collar and green lines on the sleeves, paired with a white knee-length skirt, notably worn without the neckerchief common among other female students. She leads the Y-Academy Naginata Club.

Emma's background involves a mystical origin as the opposite-gender reincarnated counterpart of Lord Enma, a significant figure from the Yo-kai World, his spirit transported to a new body following his defeat and the Yo-kai World's destruction.

During the events of the Y School Heroes movie, Emma became trapped within the Sword Tower, one of the Seven Academy Mysteries, and was rescued by protagonist Jinpei Jiba. Simultaneously, her father Kinya was ensnared by the Oyanome, another academy mystery, leading to an emotional reunion where they hugged. Following this rescue, Emma developed a secret crush on Jinpei and later observed him during a large-scale fight involving cleaning equipment among students.

Emma shares close familial bonds with Kinya Daiōji and Tamayo Daiōji. She harbors romantic feelings for Jinpei Jiba. Her favorite food is chocolate daifuku.