Sir Mordred is the illegitimate son of King Arthur. He attempts to usurp Arthur’s throne, and the two meet in battle, where they deal one another mortal blows almost simultaneously.
In most stories, his mother is a close relative of Arthur, either his half-sister (The Mists of Avalon; Hawk of May) or his aunt (The Once and Future King). His mother is usually Morgause, the wife of King Lot (and either his half-sister or aunt). But in The Mists of Avalon, Mordred’s mother was Morgan le Fey, who left Mordred to be fostered with her sister Morgause, thus fitting in with the legend.
In works prior to 1980, Mordred was almost always decieptful, and often cowardly. But in many modern novels, perhaps in an effort to go in directions not previously covered, Mordred has been treated as honorable and largely a victim of his circumstances.
One modern book that paints a sympathetic Mordred is The Book of Mordred, by Vivian Van Velde:
As is clear from the cover, Morded is the romantic protagonist of the story.