Mai Amatsu is one of the twin protagonists from the anime series Twin Angels. She is the younger twin sister of Ai Amatsu, and together they are descendants of a celestial Buddhist angel, or Tennyo, and a fisherman. This lineage places Mai in the role of a virgin priestess, a position that comes with an ancient and burdensome duty. Her family history is marked by tragedy, as her parents died when she was very young, leading to her and her sister being raised by their adoptive grandmother, Genshu Amatsu, who provided them with a loving and nurturing environment despite the strict training that awaited them.

Physically, Mai is a teenage girl with shoulder-length brown hair, which distinguishes her from her sister Ai who has a long ponytail, and purple eyes. Her appearance is noted for being that of a pure and beautiful maiden, befitting her sacred role. In her daily life, she is a student at the private Tenjin Girls High School, where her exceptional martial arts skills, combined with her approachable and friendly nature, make her a popular figure among her classmates.

In terms of personality, Mai is portrayed as a gentle and kind-hearted girl who shows consideration for others and expresses her warmth through her actions. Unlike her sister who may be more composed, Mai is described as being more fickle and easily influenced, particularly when it comes to handsome boys, to whom she is deeply attracted and prone to falling for easily. She also has a childish and clingy side, looking up to her older sister Ai and preferring to stay by her side. Occasionally, her gentle disposition means she can be a little absent-minded or naive.

Mai's central motivation is her sacred duty as a guardian. Alongside her sister and grandmother, she is tasked with protecting a boy named Onimaru, a distant relative who carries a dual potential within him: to become either a great warrior of light or a demon king of debauchery. This mission stems from a betrayal in her ancestral past, where the spirit of a betrayed younger brother seeks to corrupt Onimaru into a depraved demon lord to unleash chaos upon the world. Mai fights to prevent this dark destiny and to counter the demonic forces, known as demonic minions or the Pleasure Underworld, that serve this vengeful spirit.

Her role in the story is that of a primary protector and warrior. To combat these sexual perversion-spreading demons, Mai wields a naginata as her weapon of choice. She has been trained in martial arts from a young age, a discipline she was encouraged to pursue by her grandmother. Her abilities are further augmented by her mystical heritage; she is bestowed with the celestial robe, or Tennyo robe, which she can shrink and wear as a bracelet. By performing the Dance of the First Heavenly Robe, Mai can transform, allowing her to fight more effectively as she swings her naginata against evil forces.

Mai's key relationships are central to her character. Her bond with her twin sister Ai is the most significant, as she deeply admires and clings to her. Their grandmother, Genshu Amatsu, is a loving but firm authority figure who raised them and revealed the truth of their family's mission. Alongside Onimaru, they are also aided by his guardian, Dekunobo. Throughout her development, Mai faces considerable trials. Her pure but vulnerable nature makes her susceptible to corruption. In the course of her mission, she is captured by evil forces and subjected to various forms of torment. Despite her resistance, she struggles with her own sensitivity to pleasure, which creates an internal conflict as she strives to remain pure and fulfill her duties as a chaste guardian.