This page may contain spoilers for the whole novel.
沈巍 Shěn Wēi
Shen Wei is an immortal entity. His name has been given to him by Kunlun, the mountain god.
The first time Shen Wei appears, in chapter 3, he is described as "lean and tall, very formally dressed in a white, long-sleeve shirt and pants even in the heat of summer." He is wearing rimless glasses and presents a "cultured and pristine image."
Relationship with Kunlun / Zhao Yunlan
Shen Wei is obsessed with Kunlun / Zhao Yunlan. He has been watching him through many reincarnations, usually without him noticing. He keeps a worship room of him in his apartment. All it contains are pictures and photographs of one man, from old to new, arranged chronologically, only his clothes changing through dynasties and periods, but the man always looks like Zhao Yunlan. (chapter 47)
鬼王 Guǐ Wáng (鬼 guǐ = ghost, 王 wáng = king)
In the past, Shen Wei was called the Ghost King. He is a demon, but - along with his brother, Ghost Face - was named a king among demons.
Kunlun called him Little Ghost King (小鬼王 = Xiao Gui Wang). He fell in love with Kunlun and swore to protect him.
The Ghost Slayer
斩魂使 Zhǎn Hún Shǐ (斩 zhǎn = to behead, 魂 hún = soul, 使 shǐ = envoy)
Shen Wei is also the Ghost Slayer, responsible for judging, punishing, and slaying souls. The Ghost Slayer is described as being a creature of the Underworld, neither god nor ghost, neither spirit nor angel. (chapter 13)
He judges crimes of ghosts and when he finds them culpable, turns them into black smoke, and absorbs them. (chapter 18)
He is "completely wrapped in a black cloak. His limbs are all covered and his face is obscured by a cloud of black mist." (chapter 18) He has a "ghost-slaying blade with the ability to know 'good from evil,' which earned him his name." (chapter 13) With the black cloak and blade, he looks somewhat like the Grim Reaper, and a possible alternative translation of his name would be Soul Reaper.
His face is usually not visible.
His voice is described as "soft, pleasing and polite, but incredibly sombre." (chapter 18)
His smell is described from Guo Changcheng's point of view as mild, and not unpleasant: "The smell reminds Guo Changcheng of winter in the Greater Khingan mountain range." He compares it to the clean smell of snow, mixed with a hint of decay. (chapter 18)
Cold spreads wherever he goes, e.g. in chapter 18, where his arrival is heralded by "a rush of eerie frost," the temperature plummets, and "white fractals crystallise on the window panes." (In the novel mythology, it is very cold in the Underworld.)