Karyū

was the leader of the Dark Thunder Group.

Background
Karyū was born into a clan of Lava Release users. Unlike villages such as Kumogakure where lava users are well-regarded, Karyū lived in a small village where they were only ever treated as disposable weapons of war. For this reason, his clansmen tended to die young. Karyū did not want that fate for himself and so fled his village. His village pursued him and would eventually have found and killed him, but first he was found by Fūshin.

Fūshin invited Karyū to join his Lightning Group, which was comprised of kekkei genkai users who wanted to protect the downtrodden in ways that they had never been protected. As a member of the Lightning Group, Karyū robbed criminals and dispersed the plunder to small settlements such as Bamboo Village. The Lightning Group was regarded as heroes by those they helped. When the Lightning Group eventually fell prey to a scheme by Kirigakure and most of their members were killed, Karyū and the few other survivors fled to these settlements they'd helped. The settlements refused to provide them refuge, fearful of Kiri's retaliation; unable to find aid, one of Karyū's comrades died from his injuries.

The Lightning Group dispersed and its members went into hiding. Karyū led a despicable existence, likening his life to that of a rat as he tried to avoid detection by the Kiri-nin still hunting him. Then one day he heard of Sasuke Uchiha's attack on the Five Kage Summit. Karyū was amazed that a kekkei genkai user like Sasuke could oppose the hidden villages so openly and decided to follow his example. He established the Dark Thunder Group, its members intent on doing whatever they wanted and exacting vengeance on those who'd wronged them. For his part, Karyū led the Dark Thunder Group against the villages who'd refused to help the Lightning Group.

In the anime, Karyū was a bodyguard of En Oyashiro. He helped Fūshin establish the Lightning Group after they fled Oyashiro's service.

Personality
Karyū is described as a vengeful, cruel person, not showing mercy towards the people who weren't directly involved in his downfall. Sasuke notes that he resembles his old self.

Karyū also showed an obsession towards Sasuke, idolising him as his "master", though he also wished to surpass him by killing him. In the anime, Karyū is seen as a bit of a coward, as he was afraid of Sasuke after their fight.

Appearance
Karyū has long bluish-purple hair (red in the aniime) and pale lips.

Abilities
By combining the Earth and Fire natures, Karyū was capable of using the Lava Release kekkei genkai. With it, he was able to create a black vulcanised rubber-like material that took the form of spherical projectiles and defensive walls.

Sasuke Shinden: Book of Sunrise
Karyū and the Dark Thunder Group attack a number of villages around the Land of Hot Water, destroying the buildings and killing the villagers. During an attack on Bamboo Village, Karyū is met by Sasuke Uchiha, who has been hired to stop the Dark Thunder Group. Because he's been such a fan of Sasuke's, Karyū is well-acquainted with his abilities and takes steps to avoid his genjutsu and neutralise his lightning. Though Sasuke is briefly placed on the defensive, he eventually defeats Karyū and captures the rest of the Dark Thunder Group. He intends to turn them over to Konoha, which angers Iō, whose daughter was killed in one of the Dark Thunder Group's attacks. Iō sets the shed where Karyū and the others are being detained on fire, killing himself and the entire Dark Thunder Group along with him.

In the anime, Karyū is hired by Fūshin to impersonate him and implicate himself in the exploding humans in order to draw Sasuke for Chino and Fūshin to kill him. He is blamed by the Bamboo villagers for the exploding humans and then appears at the Coliseum long enough for Sasuke to chase after him. When they arrive at the Land of Hot Water, Karyū and Sasuke fight but Sasuke easily defeats him and threatens to kill him. Karyū surrenders and Sasuke sees through his memories with his Sharingan, discovering the truth. Karyū runs away when Fūshin appears to confront Sasuke. After Chino and Fūshin surrender to Sasuke, they and Karyū are sent to jail but are offered to work for the Fifth Mizukage as part of Kirigakure's atonement for betraying them, and all three accept.

Trivia

 * When written as "加硫", Karyū's name means "vulcanisation"; a reference to his kekkei genkai.