Ser Gregor Rivers is a knight of House Frey and the bastard of Lord Randyll Frey.
Gregor was born to an unknown northern woman. Hated and despised by his father he was sent away from the Twins, becoming a hedge knight for a number of years.
At the Tourney of the Twins Gregor entered the joust as a mystery knight, winning every match. Upon winning the tourney Lord Randyll ordered him to remove his helm and upon seeing that it was his bastard son allowed him to return to the Crossing, though still treating him indifferently.
During the Ascent of the Lion, after Randyll's capture during the failed siege of Harrenhal, Gregor killed his father, stabbing him in the chest with a dagger and assumingly escaping into the Riverlands.