Hiei is one of the shortest characters, at only 4 feet 10 inches, not including his hair.

He wears a white headband that conceals his Jagan Eye, which is placed on his forehead and has a purple iris. His eyes are reddish in tint, very large and slightly slanted, giving him in the beginning of the series a very sinister look. He normally wears a black cloak-like jacket and pants, with a black (and on occasion blue) undershirt. His hair is black with a blue tint, and usually spiked with white highlights on the front. Hiei wields a sword, which he can use both as a combat weapon and as a way to safely channel the dark flames from the underworld which he rarely uses as he finds it "barbaric". On top of a very well-muscled frame he has high speed, accuracy and precision.

Hiei is a male fire demon born to a race of female ice demons. His mother, Hina, is forced to abandon him because koorime prohibit male children. Hiei is taken in by a group of thieves, and he learns to kill at a very young age. Throughout his childhood, his one shred of sanity is a necklace made from his mother's hiruiseki (氷涙石, ice tear stone). He eventually loses the necklace in the heat of battle. In order to find the lost hirui, Hiei acquires the Jagan. He uses the Jagan to seek out the place of his birth and learns that his grief-stricken mother committed suicide; Hiei also learns that he has a twin sister, Yukina, who has disappeared from the village. He now has another goal, to search for Yukina. Hiei is aloof, elusive, and misanthropic, often belittling and mocking humans and their emotions.

Although relatively taciturn, he occasionally squabbles with and insults his teammates, especially Kuwabara but deep down he holds a certain amount of respect for him and vice versa. In fact, Hiei respects all his teammates, but seems to be closest to Kurama and Yusuke. Hiei prefers to work on his own agenda, and has a notorious reputation in Demon World as a killer even when he was a D Class demon. Despite this, deep down he is a loner that wants true friendship and companionship. He is very honorable, always keeping his word.