Rokkaku Ryūdōin

and his brothers, Genshō and Jakō, are introduced in the anime-only Third Great Beast Arc.

Third Great Beast Arc
They seek revenge on Might Guy for their father's humiliation and death. He and his brothers Manipulated Wooden dummies to attack Guy, which failed due to Guy's great taijutsu skills. It is then revealed that Guy had great respect for Rokkaku's father. Guy said that Rokkaku's father was a great and brave man who wished to fight Guy again one day. The brothers all feel like they misinterpreted their father's departing words. Sakura then shows up with a group of medic-nin.

Video games
Rokkaku made an appearance in the Naruto: Path of the Ninja 2 video game. In the video game, he and his brothers are trying to conquer the world, using the Spirit Beast. With the Spirit Beast's evil powers, he and his siblings would force others to fight against each other. In the Frontier Volcano, Gaara kills Rokkaku by lifting him with Sand Coffin and chucking him into the lava.

Quotes

 * "They are fighting each other without even knowing it, how father would have enjoyed this."

Trivia

 * Rokkaku's name means "Hexagon" or "Hexagonal".