Garnysh of the Mount
Garnysh was sitting by a tree, crying because his lady, daughter of Duke Harmel (Harniel), had not kept a rendezvous. Sir Balin stopped him from killing himself with his sword and took him to Duke Harmelís castle to find his lady. There, they found the lady sleeping in the arms of another knight, at which Garnysh killed them both, cursed Balin for showing him the sight, and killed himself.
The story appears in the Post-Vulgate Suite du Merlin, but the knight is unnamed.