Ko Hsuan
(Also known in the now-preferred pinyin orthography as Zhōng Kuí, 鍾馗: and known in Japanese as Shōki.) The Chinese wizard responsible for the seminal Taoist work The Classic of Purity: now largely held in Chinese mythological usage as a "spirit Immortal" who has become the special patron of literature, examinations and civil servants, though he was most famous during his worldly lifetime as a warrior against demons, troublesome ghosts and evil spirits. He lived during the Wude era, approximately AD 618-627.
Ko Hsuan's specialization as a wizard was mostly concerned with understanding the barriers or branes that separate the physical and nonphysical universes, and codifying the qualities of those universes. He was set on the path of this extremely abstract research during the early years after his Ordeal, when he discovered he had an aptitude for laying unquiet spirits and dealing with poltergeists and similar persistence manifestations: hence his sobriquet "the Ghost Catcher" or "Demon Queller". Zhōng Kuí's continual investigations of the boundary zones or liminal states between human life and death, and of the thousands of exoplanars or inhabited planes surrounding the merely physical universes, occupied his entire lifespan and left many people uncertain not only whether he was still human, but whether he'd ever been human in the first place.
Endless legends sprang up about Ko Hsuan over his lifetime, many of them becoming confused with the wizardly exploits of his middle life. In particular, a brutal and terrible sacrificial wizardry he enacted in mid-career on the steps of the Emperor's palace was the source of a legend that he committed suicide after failing a civil service exam. His later return "from Hell" -- in this case, actually a neighboring and fairly senior liminal state -- with an army of three thousand shining and militant spirits intent on the destruction of a plague of demons then infesting his region of China, suggests that something was going on besides suicide.
In popular culture Zhōng Kuí soon became famous as a spirit or ghost himself (that being the simplest way for his nonwizardly contemporaries to parse his return to the world of the living) and rose to the position of Protector of the Emperor's Dreams. The Emperor in question (Xuanzong) described the extraordinary figure which had cast the demons out of his personal dream landscape to his court artist Wu Daozi (吴道子), and commanded that Zhōng Kuí's portrait be painted so that it could be hung up to scare off the less dangerous demons that might possibly wander into the Emperor's daylight life. This began the tradition of hanging pictures of the Demon Queller in one's house at the time of the Children's Festival (formerly the Boys' Festival), to protect the children of the house from wandering demons. This tradition continues to this day, and has spread outside of China as well; images of Zhōng Kuí in his "Shoki" persona adorn many gateways and perch above doorways of the city of Kyoto in Japan. Zhōng Kuí is also still honored in Chinese tradition as the all-time exorcist par excellence. At least one scroll purporting to contain spells he wrote -- the Extermination of Evil (Hekija-e) or Exorcists Scroll -- is preserved in the Tokyo National Museum.
After his return from the dream realms, Zhōng Kuí went on with his research into "life beyond life", and his contemporaries report that there was nothing particularly ghostly about him as he did so. Some of them (such as his nephew Ko Hung) did mention that he seemed to have achieved amazing physical mastery over his body and his immediate surroundings. Ko Hung writes that "when (Ko Hsuan) was plastered, or sometimes when the weather was just too hot, he would throw himself into the nearby lake and just stay down there at the bottom of it for hours."
There are no traditions marking Zhōng Kuí's final death or transition into a new phase of existence -- possibly because he had been back and forth over "the line" so many times that people stopped watching to see if he was going to come back.
(See also: M-theory.)