Like Gawain, the character of Yvain allows the poet to focus on themes of morality, purity, and what makes a good and chivalrous knight. In this text, all of these themes are examined in the context of courtly love and Yvain's approach to women and marriage. At the start of the poem, Yvain appears to be a positive model of a knight: strong, brave, honorable, and loyal to his family. What becomes clear as the poem progresses is that although he appears on the surface to be a good knight, this is not really the case. Yvain's wife, Laudine, eventually banishes him for being "untrue" and failing to keep his promise to return to her within a year of leaving to go to the tournaments with Gawain. It is notable that, at this stage, Yvain is already performing chivalric deeds and heroic adventures, but for his wife, that is not enough: he is not behaving as a knight should in his heart. Both knights themselves and readers of the story are meant to realize that superficial appearances can be misleading.

The coming-of-age or self-fashioning of the ideal knight is a central theme of Yvain’s story. Although Yvain is already a knight at the beginning of the tale, he must develop his knightly good character through trials and contests. These contests test moral resolve as well as mere physical prowess. While physical combat tests Yvain's strength and bravery, his vow to his wife to return after a year tests his moral character as a knight—especially the important moral virtue of his ability to keep his vows. His failure and subsequent madness are part of his moral education. As he gradually redeems himself and reclaims his lady, he becomes a good knight in moral character as well as appearance. The maid Lunete is also an interesting character in this regard, remaining loyal and steadfast and bringing about both the central marriage and the reconciliation of the narrative.

Yvain's adventures early in the poem are simply youthful enjoyments to him, and he is privileging his own entertainment and time with his friends over his devotion and promises to his wife. He may be a knight in his own mind, but in his heart, he has not yet learned the importance of faithfulness as a knightly quality. Subsequently, Yvain must then find a way to win back his wife, which he does by befriending a lion. With the lion by his side, he is accordingly lion-hearted and is able to defeat a giant, among other foes. Finally, he rescues Laudine's maid servant from being burned to death, and she, seeing that he has redeemed himself, helps convince Laudine to take him back.

Yvain's speech toward the end of the story underlines the moral and key theme:

One ought to have mercy on a sinner. I have had to pay, and dearly to pay, for my mad act . . . if you will deign to keep me now, I never again shall do you any wrong.

To be a true knight, then, it is important to remember one's promises, and real chivalry means honoring one's wife as well as one's king and companions. The lion, Yvain’s constant companion after the knight rescues him from a serpent, is also portrayed as displaying great virtue in his loyalty to Yvain, and Yvain's character is revealed in a positive light in his relationship to the lion. This brings out the idea that loyalty is a reciprocal virtue bringing together weak and strong, human and animal, male and female, and ruler and ruled in symbiotic relationships. The combination of reciprocity and hierarchy in these relationships illustrates the medieval concept of the "great chain of being," which links together all people and creatures.