Four Knights of Gwyn

The Four Knights of Gwyn were Lord Gwyn's most accomplished warriors. Only one knight is encountered in the original console version (with no DLC installed) of Dark Souls: Ornstein the Dragon Slayer. The other three knights, Artorias the Abysswalker, Lord's Blade Ciaran, and Hawkeye Gough, are only mentioned; Artorias is deceased by the events of the game, and the fates of the other two are left unrevealed, although they are presumed deceased as well.

The PC version of Dark Souls, Dark Souls: Prepare to Die Edition, comes with extra content that takes place in Oolacile 300 years before the events of the game, and features the other three knights. The content was later added as DLC to the console versions of the game as Artorias of the Abyss.

Dragon Slayer Ornstein
According to the item description of Ornstein's Armor, Ornstein is believed to be the captain of the Four Knights. Ornstein is the only one of the Four Knights that must die by the player's hand in order for the story to progress.

The item description of Smough's Hammer reveals that Smough wanted to join the Four Knights, but due to his habit of practicing cannibalism he was not allowed. Despite this, he guards the illusion of Gwynevere, Princess of Sunlight alongside Dragon Slayer Ornstein in Anor Londo.

Artorias the Abysswalker
Artorias, as one of Gwyn's four knights, was a fierce fighter. He was the first human known to traverse the Abyss. Without the DLC, Artorias is only scarcely mentioned in item descriptions. However, in the PTDE DLC for Dark Souls, Artorias is fought as a boss. He has been corrupted by the Abyss and so he has gone mad. Hawkeye Gough will thank the player for setting Artorias free with honor. Artorias' greatest companion was Great Grey Wolf Sif who fought along side Artorias in the Abyss. Artorias saved Sif from corruption with his Cleansing Greatshield and then was corrupted by the Abyss.

Lord's Blade Ciaran
Lord's Blade Ciaran was part of the Lord's Blades, a group of assassins. Known for her unique porcelain mask, she was rumored to be the lover of Artorias the Abysswalker. She is found in the arena where the player fought Artorias. She will ask the player for his soul if the player has it. If his souls is given to her, she will give the player the Gold Tracer and the Dark Silver Tracer. Killing her will give the player both tracers and the Lord's Blade Set.

Hawkeye Gough
Hawkeye Gough was the leader of Gwyn's archers. By the time the player meets him Gough is blind. Gough will be found after the fight against Artorias the Abysswalker up some stairs. The Crest Key will be needed to open the door to Gough's tower. Gough was still able to be a great sharpshooter, proving himself by shooting Kalameet out of the sky in order for the player to beat him. Gough is one of the only two of Gwyn's knights that the player isn't forced to kill, the other being Lord's Blade Ciaran. Ranked among Gough was Pharis, which is impressive because Pharis is only human.

Rings
Their rings can be found throughout Lordran, and their item descriptions give minor insight into who they are and what they did.

Leo Ring
Dropped by Ornstein if the player kills him last.

Wolf Ring
Found in Darkroot Garden.

Hornet Ring
Found in the area of the Great Grey Wolf Sif boss battle.

Hawk Ring
Found in Anor Londo.