The Stranger, also known as the Stranger from Another World, is the name used to generally refer to the protagonist (the player) of the Ultima series. After Ultima IV, however, this hero was specifically called the Avatar.
From Ultima I through Ultima IV, the Stranger could be male or female, or "other" in Ultima III, under any name the player wished. Before Ultima IV, the Stranger was not always human; the player could choose from several races. In Ultima IV, the Stranger completed a series of quests to become the Avatar, the embodiment of the Eight Virtues and the true hero of Britannia.
In Ultima IV, it was implied that the Quest of the Avatar was the very first adventure for the player, and the manual even suggested that the heroes who defeated the Triad of Evil might have been different persons each time. Neither Ultima IV nor Ultima V make any reference to the Avatar having faced the evils of the Age of Darkness either. However from Ultima VI onward, it was clearly retconned that the Avatar was the very same person who faced Mondain, Minax and Exodus, thus creating a stronger continuity with the Age of Darkness trilogy.