Sagum holds the titles of First Prince and Crown Prince of New Yogo, being the sole son of the Mikado and the First Queen. This makes him the older half-brother of Chagum, the Second Prince born to the Second Queen.

In the anime, Sagum shares a close bond with Chagum. The brothers frequently spend time together in the palace gardens, away from their studies, with Sagum demonstrating deep care for his younger brother. When Chagum is presumed dead and his belongings ordered burned, Sagum personally takes charge of the task, secretly preserving his brother's possessions as an act of loyalty and affection. This news profoundly affects Sagum, contributing to a visible decline in his health.

Conversely, the novels initially portray Sagum as aloof and distant towards Chagum, showing no sign of a close relationship. However, later novels suggest their bond might have been closer than first indicated. Across both mediums, Sagum is heavily burdened by his responsibilities as Crown Prince. This burden leads to his death from overwork in the anime, while the novels attribute his death to illness. His demise occurs before he can reunite with Chagum.

Sagum's death has significant consequences, leaving Chagum as the sole heir and intensifying efforts by figures like Shuga to locate and return Chagum to the palace. In his final moments, Sagum releases a pet nahji bird, a symbol of his connection to Chagum, just as his tutor Gakai discovers his passing.

Physically, Sagum stands approximately 1.45 meters tall with black hair and blue eyes. He resides in the first palace of the Imperial Family and receives tutelage from Gakai, reflecting his status as Crown Prince.