Illumi generally shows very little expression. In contrast to his expressionless countenance, he often speaks in a somewhat cheerful, distracted tone. He is a cold, impassive person with a ruthless mentality, thinking only of his own interests. Yet, he holds a twisted, overprotective love for his younger brother, Killua—reaching the extent of using his Nen ability to control him to ensure his survival and obedience. He and his father raised Killua to believe that he was only supported by darkness.