Menalque seems to be a foil for the main character, Michel. In the novel, Michel is portrayed as a man who is torn between what he believes to be right and what he believes to be his prerogative to self-discovery.
In the beginning of the novel, Michel does everything a good son ought to do. To please his father, he marries Marceline, a woman he does not love. Even though he eventually allows himself to become attracted to her, he finds that he is hesitant to show his wife his true self, his innate sexual attraction to young boys and virile, young men. Instead, he dissimulates, rationalizing that this dissimulation actually fuels his sexual attraction for her. In the end, he becomes comfortable with this duplicity:
So then, as is always the case when one overcomes an initial disgust, I ended by taking pleasure in my dissimulation itself, by protracting it, as if it afforded opportunity for the play of my undiscovered faculties. And everyday my life grew richer and fuller, as I advanced towards a riper, more delicious happiness.
Menalque, the complete hedonist, is the one who challenges Michel to live his life honestly. The character of Menalque is actually based on Oscar Wilde, the homosexual author who impressed upon Gide the necessity of dispensing with dissimulation and subterfuge. Menalque's hedonism, cosmopolitanism, and repudiation of a life lived according to established principles evoke Wilde's own definition of fulfillment. So, in that sense, Menalque represents Wilde's voice in this novel; Gide is using both Michel and Menalque to tell his own story and the story of Wilde's involvement in his friend's transformation from family man to Nietzschean immoralist.
I like life well enough to want to live it awake, and so, in the very midst of my riches, I maintain the sensation of a state of precariousness, by which means I aggravate, or at any rate intensify my life. I will not say that I like danger, but I like life to be hazardous, and I want it to demand at every moment the whole of my courage, my happiness, my health... (Menalque, pp 121)
Like Michel, Gide struggled between his explosive sexual desire for his wife and his fascination for virile, young males. In the novel, Gide is forced to question the wisdom of Menalque (and perhaps, Wilde) in asking whether the repudiation of traditional codes of conduct and one's obligations constitute a fair price to pay to achieve one's uniqueness? Ironically, Gide himself excoriates his own character, Michel, for being so despicable:
He is not free; he is anarchical. [. . .] And what does he do, great gods of Greece? He shaves off his beard; he debauches while debauching himself; he covers himself with vermin; he kills his wife.
Menalque criticizes 'the laws of imitation' and the 'laws of fear' as a sort of 'moral agoraphobia' where we despise the part of ourselves which is unique and which gives us value. He sneers at those who imitate happiness by conforming to established dogmas and doctrines. Yet, Menalque's role in this novel is also to illuminate for us how to reconcile our uniqueness with our happiness. This, Michel never ends up successfully doing. He chases after the strong and the virile, leaving his sickened wife to endure physical suffering alone, and generally makes a mess of his life. When his wife dies, he is left bereft and stricken with grief. We sympathize with his untenable position.
As a foil, Menalque's role in this novel is one of illumination, inquiry, and judgment. He illuminates for us the hero's conflict with himself, inspires us to question whether our uniqueness has been submerged by societal expectation, and invites us to judge whether Michel really understood the meaning of true self-realization. As we read, we acknowledge the fact that Menalque himself realized his true uniqueness without sacrificing anyone else's happiness for his own. Wilde's Menalque is juxtaposed with Gide's Michel to invite us to question what it means to be truly unique.