In contrast to the two idealists, Roxane and Cyrano, we have De Guiche. He is a worldly, sophisticated cynic. He is motivated by personal desires rather than ideals. His own comfort means more to him than any noble idea. He takes revenge on Cyrano and Christian for having stolen Roxane from him by sending the regiment to almost certain death.
De Guiche would use anything at hand — power, influence, position, wealth — to get what he wants. He wanted Roxane because she was so beautiful; he would certainly not have married her with the idea of remaining forever faithful and devoted to her, even if he had not already had a wife. What he wanted was a rich and beautiful mistress.
De Guiche is the only character in the play that changes or develops, with the possible exception of Christian. In the last act, De Guiche has mellowed considerably, to the extent that he has developed respect for both Roxane and Cyrano. He has learned to respect their spiritual values, though he does not completely share them, and he has learned that worldly rewards are not everything.
If Rostand had shown De Guiche undergoing a greater change than this, we would be very suspicious; if he had shown him not changed at all, we would be a little disappointed. De Guiche is not able to adopt these ideals for himself, but he no longer is contemptuous of them in others.
De Guiche admits that Cyrano is probably a happier man than he is, though to all appearances De Guiche has everything a man could want. He is concerned about Cyrano and the threats to Cyrano's life: This is the same man who once sent Cyrano's entire regiment to what appeared to be certain death.
De Guiche has always had qualities of intelligence, wit and courage, but he lacked the nobility of character and dedication to ideals that Cyrano had. He is honest enough to admit that Cyrano may well have been right to choose the ideal, while realizing that the life of dedication to the ideal is not for him to embrace.
In De Guiche, Rostand has treated a living, evolving human rather than a type. He is the villain of the piece, because he does try to win Roxane by devious means and because he does take his revenge. Yet he does not seek to evade the fire of battle himself, and he grows to admit that he was wrong, which takes some nobility of character.
If Rostand had given us a villain who was as purely villainous as Cyrano was a purely heroic hero, what a boring play it would be! The fact that De Guiche does develop and does show a brave spirit under fire contributes to the believability — if not to the realism — of the play. Also, with De Guiche's admission that his own views of life might be wrong, we have further subtle support in believing that Cyrano's views might be right.