Hisoka died at the age of 16 after suffering for three years from what appeared to be an incurable illness. It turns out that he witnessed a crime at age 13, and was assaulted and cursed by the twisted Dr. Muraki. After Hisoka's death, he became a shinigami, a worker in the Judgement Bureau of the Ministry of Hades. Along with his partner, Tsuzuki, Hisoka would go down to Earth to investigate anomalies in the list of people scheduled to die. Tsuzuki and Hisoka faced off against Muraki any number of times, and he would torment them ruthlessly.
Hisoka is caustic, aloof, and closed. He has an empathic ability that lets him experience the emotions of others, an ability heightened by touch. Because of this, his parents locked him in the basement. His family is cursed, but he died before the curse could pass to him. He learned traditional Japanese martial arts growing up, but has a feminine, frail appearance. When Muraki's curse activates, red calligraphic lines appear on his skin. He suffers from nightmares and does not trust people easily. He hates looking weak.
Hisoka found himself in Avoria, and discovered that he seems, for all intents and purposes, to now be alive. He can no longer use any of his shinigami powers, like teleportation or regenerative abilities, but his empathy remains, as do minimally powered psychic defenses that manifest when threatened. His curse marks still appear, but they are no longer triggering the "illness" that led to his death.
His first time in Avoria, Hisoka developed a romantic relationship with his partner, Tsuzuki, as well as several friendships. He was continually embarrassed by most of the oddities that happened in Avoria, but prefers his happiness there to his duties as a shinigami in his own world.
Hisoka reunited with Tsuzuki briefly his second time in Avoria, but his partner returned to their world soon after. Out of intense loneliness, Hisoka contacted Yuuko and got his wish granted to be able to see Tsuzuki any time he wants (by concentrating his thoughts of Tsuzuki in a little mirror). In return, Hisoka gave up his empathic abilities for Tsuzuki; if they should reunite, Hisoka will no longer be able to use his empathy to read Tsuzuki.