Haruo Satō is a transmigrated writer, also known as a great writer or bungou, whose soul has been returned to protect literature. He is an adult male with brown hair and green eyes.

In the narrative, he serves as the mentor to Osamu Dazai. A defining event in their shared past involves a literary betrayal in which Satō did not leverage his influence to secure Dazai's victory for the Akutagawa Literary Prize. This incident remains a point of conflict and a source of betrayal for Dazai.

As a transmigrated soul, his purpose is to enter tainted books and fight the Taints, entities born from negative emotions toward literature that seek to erase books from existence.