The Cobalt Knights (碧衣の騎士団) are administrators working for CC Corp in charge of debugging The World. The majority of them are equipped with a Divine Spear of Wotan, a weapon capable of deleting Vagrant AIs. As a result, they almost completely consist of Long Arms. They were known as the "Cerulean Knights" in the Legend of the Twilight anime dub.
History
.hack//AI Buster
In the early days of The World, the leader was Kazushi Watarai, who used the PC Albireo, undoubtedly the most iconic of the three leaders. A long-time player and one of the people that worked on the development of The World itself, he was completely devoted to his task. He took upon most assignments by himself and did not let a single target get away with the sole exceptions of Tsukasa and Macha. Before he could approach Tsukasa, Albireo encountered Macha, who broke his Divine Spear of Wotan and caused Watarai to become unconscious in the real world. Watarai had to be hospitalized for roughly a year.
A Cobalt Knight.
.hack//Games
While Watarai was recovering in the hospital, the Cobalt Knights would take orders from Lios, the main system administrator. Much like Albireo, Lios took it upon himself to restore order within The World. He would target the player Kite, owner of the Twilight Bracelet, as his actions seemed to be causing what would later become known as the Twilight Incident. Although Lios originally performed his tasks without questioning them, he saw first-hand the true mysteries behind The World, including Cubia, and that Kite himself was working to restore order in The World. He joined forces with Kite and Helba to defeat the threat. Although he lost all of his men in the process, the coalition succeeded in defeating the glitching Morganna system and allowing the Ultimate AI, Aura, to be reborn. Although it was good for The World, CC Corp was not pleased with Aura's rebirth.
Among the changes caused by the Twilight Incident was the exposure of the true identity of the Cobalt Knights. Because of that, the Knights were forced to disband. When Watarai recovered, he was forced to take responsibility for the Twilight Incident and resigned.
.hack//Kamui
.hack//Legend of the Twilight Bracelet (manga)
CC Corp created a copy of the original Spear of Wotan and reorganized the Cobalt Knights under the leadership of Saki Shibayama, Watarai's subordinate. Her character, Kamui, adopted the same eyes as her predecessor Albireo. She would take with her an even greater devotion and fierceness when it came to her job, especially when it involved Vagrant AIs. After over a year on the job, she targets Shugo and Zefie. Her encounters with them led to the destruction of her Spear of Wotan. Although she eventually learned something from the experience, her drive to protect The World would not diminish.