In Sir Gawain and the Green Knight, one can easily justify that Gawain only has one secret weakness in combat.
Gawain admits that he is the one who should take the challenge made by the Green Knight based upon the fact that
One can easily conclude from this that Gawain does not think very highly of himself and that, if he loses, his life will not be worth as much as the other knights in King Arthur's castle.
While this is not a true secret, given Gawain readily admits it, one can see his true inner fear (regarding combat) based upon his actions when receiving his blow from the Green Knight the following year.
But Gawain swerved aside as the axe came gliding down to slay him as he stood, and shrank a little with the shoulders, for the sharp iron. The other heaved up the blade and rebuked the prince.
Here, the true fear of Gawain's is seen. The Green Knight thought that he knew Gawain to be fearless, a true knight, a Knight of the Round Table.
"Thou art not Gawain," he said, "who is held so valiant, that never feared he man by hill or vale, but thou shrinkest for fear ere thou feelest hurt. Such cowardice did I never hear of Gawain! Neither did I flinch from thy blow, or make strife in King Arthur's hall. My head fell to my feet, and yet I fled not; but thou didst wax faint of heart ere any harm befell. Wherefore must I be deemed the braver knight."
As stated by the Green Knight, Gawain's secret weakness (his fear) has come to the surface. After being berated by the Green Knight, Gawain replies:
"I shrank once, but so will I no more, though an my head fall on the stones I cannot replace it. But haste, Sir Knight, by thy faith, and bring me to the point, deal me my destiny, and do it out of hand, for I will stand thee a stroke and move no more till thine axe have hit me--my troth on it."
Gawain, finally, is able to become the knight all others believed him to be. After "shrinking" from the first blow, Gawain is ashamed that his fear was seen.