- Shun was an only child. His family lived in the village of Pinewind. When the characters were young, they had been to Shun's house often to play; as they grew older, they played outside more often. Shun is first seen with his dog Subaru in a game of capture-the-flag on a hill near Gold, where he served as the game's referee.
- After going through the coming-of-age ritual at the Temple of Purity with the monk Mushin, Shun was the first to attend Sage Academy, as his Cantus was awakened first. He was assigned to Group 1 which eventually consisted of himself, Saki, Satoru, Maria, Mamoru and Reiko.
- During the summer camp, he encountered the false minoshiro along with the rest of Group 1 and learned the truth about the bloodstained history his society was founded upon. Shun showed distaste for their violent history, but was also the most interested in finding out more. Before they could learn the whole truth, however, they were caught by a priest from the Temple of Purity and had their Canti sealed. While on the way to the temple, the group was attacked by Queerats from the Ground Spider colony. Shun managed to hide from the Queerats and reunite with the rest of Group 1 at the place where they hid their canoes. That night, Saki unsealed Shun's Cantus.
- At 12, it is shown that he and Saki have a mutual attraction to each other, but after the two year time skip, he became Satoru's boyfriend. Their relationship was perceived to be happy and stable by outsiders, until Shun suddenly broke up Satoru, stating that there is no sense in them pretending to be lovers any longer and that he wanted to be by himself. In reality he probably wanted to protect Satoru and his friends from his leaking Cantus and the probability of encountering the Tainted Cats sent to take his life. After warning Saki about the cats, he disappeared from his home and from school. Saki and the rest of Group 1 attempted to search for him after he was missing for five days.
- When Saki later finds him, he tells her that his subconscious is continually leaking Cantus and warping the environment around him, fitting the description of a karma demon, and that he cannot be allowed to live. When one of the Tainted Cats comes for his life and kills his dog Subaru in his attempt to protect his owner, Shun decides to die than to hurt the people he loves. With little time left before he is killed by his own uncontrolled power, Shun reveals to Saki that he always loved her before having her flee to safety.
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