Karyū

was the head of the Dark Thunder Group. Sasuke Uchiha met him during his redemption journey around the world.

Background
Karyū came from a small village in the Land of Lightning. Since the member of his clan owned a kekkei genkai, Kumogakure often employed them during its battles. Seeing that he was eventually going to die as a tool, Karyū decided to leave his village and become a missing-nin.

As he was about to being captured by Kumogakure's hunter-nin, Karyū was found by the head of the Lightning Group, a kekkei genkai owner like him, who accepted him in his group. Karyū began taking part in the Lightning Group's activities, such as robbing the rich and redistributing the plundered money to the poor villages.

When the Lightning Group disbanded after being lured into a trap by Kirigakure, Karyū began living as a fugitive again, cursing the villages that refused to help him in fear of the shinobi villages' possible retaliations. After many years in hiding, Karyū came to know about Sasuke's attack at the Five Kage Summit: being impressed by the fact that Sasuke, owner of a kekkei genkai, wasn't afraid of opposing against the shinobi villages, he decided to start gathering people for his revenge against the villages that denied him a refuge, calling his old comrades back and forming the Dark Thunder Group.

After the Fourth Shinobi World War, the Dark Thunder Group began growing in size because of the recruitment of unemployed shinobi. Then, Karyū began carrying out his revenge, sacking the villages and killing all their inhabitants.

Personality
Karyū is described as a revengeful, 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.

Appearance
Karyū has long bluish-purple hair, and anaemic-looking lips.

Abilities
Karyū's kekkei genkai, the Lava Release, allows him to mould a vulcanised rubber-like material.

Sasuke Shinden: Book of Sunrise
As the head of the Dark Thunder Group, Karyū had been destroying many villages around the Land of Hot Water, the Land of Water and the Land of Lightning, including the village in which Iō's daughter lived. When he planned to destroy the Bamboo Village, he came to know that Sasuke was there as well, and decided to meet his alleged master.

After Sasuke, who had gone to Iō's daughter's village to investigate, was warned by Chino that someone was attacking the Bamboo Village, Karyū was finally able to rendezvous with the man he longed for. The two engaged in a battle, resulting in Karyū's loss despite he made use of his strongest techniques.

Upon being captured and restrained with the rest of the Dark Thunder Group, Karyū revealed Sasuke his past and the reason of his grudge towards the village. However, before Sasuke could interrogate him further, Karyū perishes in a fire that involved the whole Dark Thunder Group as well as Iō.

In the anime version, however, 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.

Trivia

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