Hiei is a hybrid fire and ice yokai. 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, using his right arm as a sacrifice, and which he later consumes to be able to call upon its power whenever.
He possesses an implanted third eye in the middle of his forehead, called a Jagan (literally Evil Eye; which is the term used in the English-language manga and Poltergeist Report dub when referring to it). 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. Yet, despite all this, Hiei must hide it behind a special sutra enscribed strip of cloth he uses as a bandana, because despite it being a part of him, the Jagan is a sentient being and has a mind of its own and needs to be kept in check by such items..
Hiei typically squabbles with Kazuma Kuwabara and keeps a cold and uncaring exterior, but seems to hold a certain amount of respect for all his teammates. He seems to be closer to Yusuke and Kurama. Hiei prefers to work on his own agenda, and has a notorious reputation in the Demon Realm, but despite this, he is very honorable and always keeps his word.