Sir Gawain and the Green Knight is a medieval virtue poem depicting the struggles of the knight Gawain. Throughout the poem, Gawain's shield, the pentangle, is used as a symbolic representation of his virtues as a knight. The five-sided shield is "an emblem of fidelity," and the poet goes on to say that Gawain embodies knightly virtue in five distinct ways. The first four ways in which Gawain is virtuous are: 

First he was found faultless in his five senses,

and then failed never the knight in his five fingers,

and all his trust in the field was in the five wounds

that Christ caught on the cross, as the creed tells.

And wheresoever this man in mêlée was stood,

his first thought was that, over all other things,

all his force in fight he found in the five joys

that holy Heaven’s Queen had of her child. 

The final side of the pentangle represents the "fifth five" expressions of Gawain's knightly virtue. This "fifth five" is often considered on its own as "the five virtues of Gawain," but in fact it connects to the preceding four in "an endless knot": 

The fifth five that I find the knight used

was Free-handedness and Friendship above all things;

his Continence and Courtesy corrupted were never,

and Piety, that surpasses all points—these pure five

were firmer founded in his form than another.

Gawain, then, according to the poet, embodies the code of chivalry better than any other knight in Arthur's court because he is brave in battle, strong in his faith, generous, friendly, courteous, continent (or chaste), and pious—this being the most important quality in a knight.

Sir Gawain and the Green Knight is part of a group of Arthurian romances which reflect a code of chivalry. The story itself is about Sir Gawain's development as a knight and what is necessary in order to be a good knight by chivalric standards. Several different things are required:

1. Physical prowess and weapon skills: Knights were above all soldiers and warriors. In order to be a good knight, one required the ability to win exhibition tournaments or jousts as well as triumphing in actual battles.

2. Piety: The tale is Christian in moral and religious orientation. A good knight must be a good Christian, pious, devout, and dedicated to upholding Christianity and following its moral precepts.

3. Chastity: The knight must resist the temptation to commit adultery and treat women honorably.

4. Brave, loyal, and truthful: The encounter with the Green Knight tests these elements of Gawain's character which are as essential to knighthood as brute strength.

 Sir Gawain and the Green Knight is a tale of a knight's passage to true manhood and maturity.

• Bravery - Sir Gawain is bold and brave in volunteering to take the place of King Arthur and accept the challenge of the Green Knight:

I pray thee [Arthur] of thy grace
Be this adventure mine!"

Unfitting do I deem that such a boon be sought....
While many a valiant knight doth sit beside thee still

• Honor and Self-discipline - Sir Gawain honors his agreement to meet the Green Knight in the Green Chapel. On his way, he comes upon a castle in an enchanted forest and stops there. He is welcomed and invited to spend the holidays there. While staying at the castle, Sir Gawain refuses the advances of the lady while Sir Bernlak is gone.
• Courage - Sir Gawain honors his agreement with the Green Knight, meeting him at the Green Chapel. Gawain bravely faces death after the Green Knight reveals that he is actually Sir Bernlak and he knows that Sir Gawain has kept the green girdle without telling him. For this reason, he gives Sir Gawain two feints, and one blow, but it just breaks the skin.

These traits of bravery, honor, and courage are what make Sir Gawain a hero. When he returns to the court of King Arthur, although Gawain later reproaches himself for coveting the green girdle, his only fault is wanting to save his own life. Ironically, this flaw makes Gawain seem more real; consequently, it becomes easier to admire his virtues of courage and knightly courtesy as the other knights do when they laugh at his saying he will wear the girdle to remind himself of his sin. They agree to also wear green girdles as a symbol of Gawain's honorable adventures:

Each of the Brotherhood, should bear, as baldric bound,
About his waist ...a badge of green so bright 
This would they fitly wear in honour of that knight.