Illumi Zoldyck is the eldest son of the Zoldyck Family. At the beginning of the series, he appeared under the guise of Gittarackur in the 287th Hunter Examination. Also, Illumi currently seems to be the only person that Hisoka claims as friend, although their relationship is somewhat unconventional.Illumi appears expressionless, his face like an emotionless mask. He has very long jet black hair, usually combed behind his head. He also has very dark eyes. To disguise himself as Gittarackur, Illumi uses large round head pins to reconstruct his facial structure, and even voice, beyond recognition. The pins in his chest seem to affect the muscles in his body, as he looks more slender without them.
When he was younger, Illumi's hair only reached his shoulders. He wore long sleeves with a shirt underneath, as well as grey pants.
Illumi, for the most part, displays very little expression. In contrast to his expressionless demeanor, Illumi often speaks in a somewhat joyful and absent-minded tone. He is generally a stony, ruthless person with an extremely cold way of thinking, reflects only his interest. Still, he holds a twisted and overprotective form of love for his younger brother, Killua; this manifests to the point that he used his Nen ability to control Killua and ensure he survives and obeys. He and his father raised Killua to believe that the only thing sustaining him was darkness, and that he could only obtain joy in causing the death of others. To ensure that Killua would live to carry on the family business, and as a means of protecting his little brother, Illumi implanted a needle in Killua's brain, forcing the latter to run from battles he wasn't sure he could win. He also has an unusual friendship with Hisoka, although it is unclear as to how it developed. Illumi once came to the aid of Hisoka by standing for him as a double in the Phantom Troupe's hideout while Hisoka attempted to find Chrollo. However, when Hisoka asked Illumi would it be okay if he killed Killua, Illumi displayed a horrifying side of himself and asked if Hisoka wanted to die then and there - he acted very similar to how Hisoka did himself after Illumi was opting whether or not to kill Gon, whom he sees as a threat due to his closeness with Killua, during the Hunter Exam, albeit on a much lesser expense.
Illumi cares for most of the Zoldyck family members. He is willing to handle Alluka's power, even if it costs him his life. He has stated, however, that family members are not allowed to kill during an inner mission (a mission where there is internal conflict), but insinuates that he would be fine with killing Alluka and does not consider Alluka a member of the family.
Tsubone implies that Illumi's sinister ambition to control Alluka's power even caused him to stray from the path of an assassin.
Illumi is the eldest son of Silva and Kikyo, and was raised from birth in the art of assassination. He trained Killua to become an assassin. Illumi was sent by his mother to watch over Killua, who also entered the Hunter Exam. In order to control the development and actions of his younger brother, prior to the series Illumi inserted a needle into Killua's brain, forcing him to run away from the battles he wasn't sure he could win. This needle was later self-extracted by Killua in his battle against Rammot.