Sir Ruber is a minor antagonist in the Kingdom Hearts Unlimited Saga, first appearing in "Return of the Keyblade" as the second-to-last boss of the Camelot/Enchanted Forest campaign.
He is a former knight from the Court of King Uther who desired the throne of Camelot and all of England with it for himself. So, he made a deal with the Wyrd Sisters to gain dark magic and a metallurgy transfiguration potion in exchange for most of his sanity. Ruber was eventually recruited by the Order of the White Rose/Golden Cross as a world agent, and with their efforts was able to recover Caledfwlch, the Sword in the Stone, but was overwhelmed by its massive power and thus met his end. He is ultimately just a pawn in Maleficent's experiments in studying both the reality-warping effects of the Book of Prophecy and the corruptible effects of chaotic darkness to the heart from chasing lures based in ambitious greed.
The son of a minor knight in the court of King Uther; Ruber was forced to endure his father's drunken ramblings, delusions of grandeur and violent abuse. While his father trained him in combat, Ruber was forced to teach himself how to read and write - he studied military strategy, reading countless tomes about tactics and the art of war.
When he came of age, Ruber joined the army of Uther's newly ascended young son Arthur and worked his way through the ranks. His bravery on the field of battle earned him a seat at the Round Table, and his grasp of military tactics make him a respected general. He devised strategies which destroyed several invading forces who sought to conquer Camelot, and the people rejoiced. When Mad Madam Mim raised an army of monsters and attacked the kingdom, it was Ruber who lead the charge and banished her to the wilderness.
Secretly, however, Ruber was jealous of Arthur's power, and believed that the boy King was taking all the credit for the victories that were rightfully his. He wanted the adoration of the people, and for all his service to the country, he saw little reward. While at court, he befriended a beautiful woman named Julianna, whom he developed feelings for. He was crushed when she later began a relationship with fellow knight, Sir Lionel.
In peace time, Ruber was bored. With the last of her enemies defeated, Camelot was free from conflict. When Arthur decided to divide the kingdom according to each person's needs, Ruber was shocked to find himself with the least amount of land. He openly challenged Arthur for control of Camelot, but was quickly defeated and forced to flee. His temper had cost him everything, and the people now saw him as a villain. Ruber decided to court this image and make himself a feared enemy of the crown - vowing that he would one day be King and claim Camelot for himself. (Cited from 73Windman's bio for the character for DVND)