Sa Sakujun is the biological son of Sa Chuushou, though publicly presented as his grandson. His family consists of half-brothers Sa Bou and Ryou Anju, nephews Sa Soujun and Sa Kokujun, and grand-aunt Hyou Eiki. At age 15, he assumed leadership of the bandit group "Murderous Blades" (Satsujinzoku), wielding them for his personal amusement. Fourteen years before the main events, he discovered the injured Prince Seien (later Seiran) in the snow and deliberately delivered him to the bandits. He possesses long, wavy red hair, distinctive amber "cat-like" eyes, and is consistently described as exceptionally handsome. He favors elegant clothing in colors that complement his appearance. His personality blends charisma, seductiveness, and charm with hedonism, ruthlessness, and sociopathy. Profoundly bored by life and devoid of regard for humanity—including himself—he engages in manipulation and underhanded murder. He exhibits intense possessiveness toward his goals, particularly concerning Kou Shuurei, aiming to remain permanently in her thoughts even after death, a shift stemming from his narcissistic mentality following their encounter. Operating under the alias "Lin Senya" (merchant's son), he orchestrated a meeting with Shuurei. Though uninterested in power or leadership, he pursued marriage to her to alleviate his boredom. To facilitate this, he manipulated Sa Chuushou into imprisoning his own son using mind-altering incense and arranged the murder of his elder brother, Sa Soujun, to position himself as the Sa clan's heir, gaining leverage against the Kou clan's potential opposition. His fixation on Shuurei intensified when she refused to prepare "ganlu tea" for him, reserving it only for those important to her. This rejection fueled his obsession to become someone she would acknowledge through the tea. In a calculated act, he poisoned hot water and placed the antidote in the tea, forcing Shuurei to choose between serving him the tea or causing his death. She chose the water, leading to his apparent demise. As he died, he expressed regret that she never used his real name and wished to hear her play the ehru more, indicating complex emotions beneath his manipulation. His body vanished afterward. Post-death, he was revived as an "empty shell" by the Black Immortal using Shuurei's life force. His body was puppeteered by Ryou Anju to destroy sacred artifacts, assassinate Hyou Ruka, and kidnap Shuurei. He temporarily regained his will during the kidnapping, aiding Shuurei's escape before his body was re-controlled. His skills include swordsmanship equal to Seiran, agility enabling evasion of projectile attacks, stealth honed from leading bandits, and immunity to most poisons due to prolonged self-exposure. His relationship with Shuurei is his sole noted emotional attachment. Initially driven by twisted possessiveness and a desire to "own" her, his actions evolved into self-sacrificial gestures, such as the poisoned water gamble and his final defiance against Anju. Shuurei, aware of his crimes, acknowledges an inability to hate him and becomes his only mourner.