The story takes place in modern-day Tokyo. Kotono Mitsuyoshi is on the train when she sees a man being hurt by a mysterious person. When she gets home, her parents tell her there is going to be a boy staying at their home. Then she meets a strange kid called Aiten, whose job is to judge those who sin.
- The shy 15-year-old main character of Aiten Ryōō Monogatari.
- The titular character. Aiten is part of a homestay program.
- A dark-skinned 16-year-old that is a major flirt and quite a lesbian bent. Chiaki spends a lot of time with someone called Miss Ohbara.
- A blonde girl who appears at first to be only Chiaki's underling, but later is revealed to be kinder and wiser. In the first chapter, Kotono meets Aiten, Ohbara-chan has a beauty-mark on her face, but it disappears afterwards.
- Kotono's young friend.