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 and reflects solely on his own interests. 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 in order to ensure his survival.