Early accounts stating that this Arthurian hero's strength increased until noon and then waned hint 
Early accounts stating that this Arthurian hero's strength increased until noon and then waned hint at his origins in solar myth.  The son of Lot and Margause, he defeats and kills his mother's lover Sir Lamerok and Morgan Le Fey's son Sir Uwayne. He avenges the death of his father by killing Pellinore with his brother Gaheris, but is killed after twice challenging Launcelot to single combat. Known as "the Courteous", he is denied the Grail on account of his sins. For 10 points--identify this Arthurian hero who beheaded the Green Knight.
Early accounts stating that this Arthurian hero's strength increased until noon and then waned hint at his origins in solar myth.  
Sir {Gawain} (accept, but be surprised to hear, {Gwalchmei, Gauvain,} or {Galvanus})