The Count of Nantes in Eilhart's Tristrant.
Riole loved Isolde of the White Hands, daughter of King Havelin of Karahes (and later Tristan's wife). When Havelin refused an offer of marriage, Riole attacked him. He besieged Karahes (Carhaix) and probably would have won the war, but Tristan (Tristram) arrived and, championing Havelin, defeated Riole. When Havelin died, Riole resumed the war against Kahedins, Havelin's son, but was again defeated by Tristan.
His counterpart in Gottfried's Tristan is Rigolin.