Hiei is the shortest of the four main characters at only 4' 10" during the Dark Tournament (excluding his hair) as stated by Juri. At the end of the series, an artbook puts him at 5' 3". His height is never mentioned in the anime or manga, aside from the DT reference. Despite his short size, he has a lean, muscular build to him. He wears a white headband that conceals his Jagan Eye, which is placed on his forehead and has a purple iris. His eyes are crimson (blood red), with an angular and slightly slanted shape, giving him a very sinister look in the beginning of the series. He has spiky black hair (reminiscent of Vegeta from the Dragon Ball franchise) with blue outlining and white streaks. His skin complexion is regular peach-tan.
Living up to his classic villainous persona, he usually dresses entirely in black. He wears a long black cloak-like jacket (with a white collar) and black pants tucked into black boots. He wears a black (and sometimes pale blue) undershirt under the jacket. He is the only one of the four main characters to never change his primary 'costume' during the series. The only exception was when he briefly wore light-colored Chinese battle costume at the start of the Dark Tournament. He frequently carries a sword with him that he keeps strapped across his back.
Before mastering the Dragon of the Darkness Flame, he kept his right arm covered in bandages and bindings to keep the dragon from escaping. After gaining full control over it, a dragon tattoo now covers the arm and is visible when the bandages are removed.
Hiei prefers to keep his distance from others, so he often comes across as aloof and elusive. He has a rather cynical and even misanthropic view of the world around him, often belittling and mocking humans and their emotions and the significance they place on friendship and life. And while he is relatively taciturn, he occasionally squabbles with and insults his teammates, especially Kuwabara. It is obvious that deep down he holds a certain amount of respect and kinship for his teammates, but seems to be closest to Kurama and Yusuke Urameshi.
As a fighter, Hiei is very unbiased when it comes to his enemies. He shows the same degree of ruthlessness to his foes - regardless of age or gender. This is seen when he terrorizes Ruka and the Masked-Fighter with threats of death as his power is returning. In battle, Hiei has killed the most opponents on the team- often making good on a threat he made at the outset. By contrast, he shows a surprising degree of sincerity, warmth, and protectiveness when around Yukina.
It's made clear in both the anime and the manga that he holds the demoness Mukuro in high esteem, perhaps higher than any other character. He respects her for her power and accomplishments. At the end of the series, there is clearly a bond between them, as it's made clear that its by his choice that he remains at her side. He also goes out of his way to free her from her past, breaking her shakle in the anime and delivering her abusive father to her as a birthday present in the manga.
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 has struggled with his identity as a loner and his abandonment.