Third Kazekage was once the leader of Sunagakure, though his true name is unknown. He had the unique ability to manipulate his chakra and convert it into magnetic forces, which made him immune to all metallic weapons. After studying the host of Shukaku of the time, he recreated its sand-manipulating powers by combining his own magnetic chakra and iron powder, which produced the Iron Sand technique. It was for this very ability that he was noted as the strongest Kazekage in the entire history of Sunagakure. Sasori kidnapped and killed the Third Kazekage in order to turn him into a Human Puppet. The effort involved in the Third's capture made it his favorite puppet. Sunagakure did not know this and searched earnestly for his whereabouts. Their actions are suggested to have sparked the Third Great Shinobi World War. With the war's outbreak the village was forced to give up the search and choose a Fourth Kazekage, though there would be other search attempts in later years. The Third's fate is discovered during Sasori's battle with Sakura and Chiyo when Sasori brings out the Kazekage puppet to use against them. After a long, drawn-out battle, Sakura managed to destroy the puppet.
Son Gokuu, more commonly known as the Four-Tails or Yonbi, is a tailed beast sealed within Roushi of Iwagakure. It was captured by Kisame Hoshigaki then extracted and sealed by the Akatsuki (Source: Narutopedia)
Izuna Uchiha is Madara Uchiha's younger brother. He was alive about 70 years before the start of the series, even before the founding of Konoha. Standing at the top of the Uchiha clan with his brother, Izuna was one of the first to obtain the elusive "Mangekyou Sharingan" (along with Madara). However, Madara went blind due to the use of his. To alleviate this, Izuna gave up his eyes so that Madara would obtain a new, "permanent" Mangekyou.
Toroi was a famed Kumogakure shinobi. In preparation for the Fourth Shinobi World War, Toroi is reincarnated by Kabuto Yakushi to fight against the Allied Shinobi Forces, before being mobilised alongside Gari, Pakura, Haku and Zabuza Momochi. Separating from this group at some point during their relocation, Toroi is later discovered trailing Musai and Nonota, with numerous shuriken in hand and scroll in mouth. Identified by the Kumo-nin, who attempts to alert his ally to the best method to not fall victim to his Magnet Release, Toroi goes on the offensive. Despite the pre-emptive warning, the unaware Suna-nin still falls foul of the attack and is evidently terrified as the details of the technique are disclosed. Prepared to conclude the battle using even larger shuriken, Toroi is ambushed from behind by Naruto Uzumaki wielding a Rasengan, before being sealed by the previously panic-stricken Suna-nin. Later, with the release of the Impure World Reincarnation technique, a light of sorts encased Toroi's sealed body as the technique was dispelled and his soul was returned to the afterlife. (Source: Narutopedia)
Kyuusuke (鳩助, Kyūsuke) was once a shinobi from Amegakure and one of the original founding Akatsuki members. (Source: Narutopedia)
F is a Kumogakure shinobi. During the Fourth Shinobi World War, F was attacked by a White Zetsu Army clone on the battlefield and was later impersonated by this clone. It is unknown if the real F is still alive. (Source: Narutopedia)
Shukaku is the demon that is in Gaara giving him most of his power and his insanity. Shukaku is able to fire off sand chunks that look like and act like kunai. In the English dub, Shukaku is made sounding like an insane cowboy. Shukaku is the Ichibi, which is the One-Tail Jinchuriki. Shukaku is very powerful as Gaara can still not fully control him.