Rob is basically Sly in the Archer Costume, but with a gold feather instead of red and a stronger Medieval accent than Dëma Luper.
The only difference between Sly and O'Coop-Er is that Rob is more clueless about things that he doesn't usually see and he has a greater sense of chivalry to women and allies.
As a cub, Arthius Cooper was to be future ruler of Parilot, being the descendant from a long line of royalty and knights starting with Sir Galleth Cooper. His father, King Commodore Cooper, ruled Parilot in the same respect he does for his son. However, a jealous knight, the Black Clock Knight destroyed Commodore on the night Arthius was to be trained as the new king. Since Arthius was too young to rule by himself, the king's advisor, Octjohn became King and exiled the young prince to a town far outside Parilot. The wizard of the castle, who was close to Arthius, enchanted a special sword called Excalibur and placed it in a stone for the true king's return.
During his exile, Arthius was found by two orphans named Pence and Mar, who both lived in the neighborhood's orphanage. Among all the children, only the two believed that their new friend was a prince from another land. The three became best firends because they believe each other.
As teens, they stole from the town's new tax collectors, who were selfish and held taxes high. It was during one of these many heists that he renamed himself "Rob O'Coop-Er". One night, he was discovered by the visiting princess, Lady Carmivere, who had ran from the palace to escape the king and her own father's selfishness. Following their shared belief of justice, the four manage to bring down the ruling king's glory and had him replaced by a more caring man. With this, Rob believed it was time to reclaim his heritage and so the gang, excluding Carmivere, went off to Parilot.