Lord British
Lord British was the King of Britannia. He was played in-game by Richard Garriott, the creator of Ultima Online.
The NPCs of UO consistently, and with apparent sincerity, discussed their love of and respect for their King, this adding to the fiction that the realm was well-governed and that Lord British (often abbreviated by players to "LB") was a benevolent ruler.
The fiction was that Lord British was somehow from the real world, as was the Avatar.
Lord British invented the Virtues. He was friends with a political rival, Lord Blackthorn, who invented a competing belief system called Chaos and would, eventually, turn to true evil. The fanatical followers of each man fought in the streets as part of the Chaos/Order PvP system, while the two men played chess amicably and discussed philosophy.
Lord British disappeared for unknown reasons after Garriott left EA. After he left this first time, Blackthorn, furious with his not being appointed to the throne in LB's stead, struck an alliance with Exodus and turned true evil.
Lord British later returned, following Blackthorn's death, around the time of the Age of Shadows expansion, for a plotline involving Minax. Garriott did not play Lord British for this return, but rather simply allowed UO to use the character on a temporary basis, to have some fictional explanation for the departed king. He left again in short order, in order to take a powerful artifact of some kind to the Ethereal Void to guard it from Minax.
In his stead, the realm is ruled by the Britannian Council, basically a Parliament of some kind, and or by the role-played High Councils that exist on various, but not all, shards.
Trivia
During UO's beta, Lord British was killed by players by use of a [Flamestrike]] spell. Accounts differ if the players used an Exploit, or if LB forgot to turn on his "invulnerable" flag. Either way, the killing player was banned, for this and for other bad things done during the beta test. This event was used in the fiction of the Artifact known as the the Royal Leggings of Embers. Richard Garriott also challenged his killer to a rematch in his more recent game project, "Tabula Rasa." So far as anyone knows, the challenged did not respond.