Illumi is generally expressionless but speaks in a joyful tone. He is ruthless and self-interested but has a twisted, overprotective love for his brother, Killua. He implanted a needle in Killua's brain to ensure his survival and obedience. Illumi cares for most Zoldyck family members, even risking his life for Alluka's power, though he doesn't consider Alluka a full family member. He desires Alluka's ability and had a complex relationship with Hisoka, described as "give-and-take." Their relationship was mutually beneficial but Illumi expected one of them to kill the other. He is a dangerous assassin who casually commits mass murder.