In medieval romances, there is often a character
whose identity is hidden or mistaken. explain how this characteristic affects
the outcome of sir gawain and the green knight.

Hidden characteristics of of Sir Gawain and the Green Knight:

Sir Gwain and the Green Knight is a medieval romance in as far as it deals with adventures of a brave and courageous knight, Sir Gawain, who accepts the challenges of a Green Knight and beheads him once with the Green Knight’s axe in King Arthur’s court as per the Green Knight’s wish.  

The condition that the green knight puts forth before giving the challenge is that he would return it in a year and a day in the green chapel. Actually, it is a game.  After he is beheaded once, he gives his head to the queen of King Arthur’s court and rides away.

In the end, the Green Knight turns out to be Bertilak, the lord of a castle that Sir Gawain visits on his way to the green chapel  and stays on in on the request of the lord.

He is transformed into the Green Knight by magic of King Arthur’s sister, a sorceress who wanted to test Arthur’s Knights. He is the hidden character who reveals his true identity in the end after Gawain overcomes his trials.  

Gawain is saved from the Green Knight’s blow because of the girdle gifted  to him by Lady Bertilak. In the end, Lord Bertilak calls him a blameless Knight in the whole land.

so is this a rhetorical question?