Hiei is a fire demon born to an ice demon clan that abandoned him upon his birth due to his difference. He typically squabbles with Kuwabara and keeps a cold and uncaring exterior, but seems to hold a certain amount of respect for all his teammates. Hiei can be kind and compassionate, but really doesn't know how to show such things because of where he grew up and the atmosphere there. From a very young age he learned to care for himself, and only himself, but didn't completely close off his emotions as he is still able to care for others.

He has some pyrokinetic and cryokinetic powers, which grant him amazing resistance to extreme temperatures as well as limited immunity against fire and ice based attacks. Other than being a master swordsman, Hiei is also lightning fast, able to move much faster than human and sometimes demon eyes can follow. He is a master of the Kokuryuuha, or Dragon of the Darkness Flame, which he tamed and summons from the Demon Realm for his use through the mystical tattoo on his right arm. The tattoo was created when Hiei merged with the dragon in preparation for the finals of The Dark Tournament. It is actually a permanent burn that was scorched into his arm by the awesome power of the dragon.

He also possesses an implanted third eye in the middle of his forehead, called a Jagan (literally meaning ‘evil eye’). Said implant amplifies all his natural powers, and enables him to summon both the hell-fires of his homeland, Makai (Demon Realm) and the tamer, but still dangerous, fires of Ningenkai (Human Realm). At the same time, it grants him several near psychic abilities, such as: Hypnosis, Telepathy, and Remote Viewing powers. Also, the Jagan allows Hiei to see quick movements.

One reason Hiei received and uses the Jagan eye is so that he can watch over his twin sister, Yukina, who has yet to know about their true relationship.

It was also revealed during the Dark Tournament that Hiei's name means 'flying shadow', which is fitting due to his incredible speed that would have aided him in the few fights he was in.