The two central characters of "The Merchant's Tale" are husband and wife, named Januarie and May. Throughout the course of Chaucer's exploration of their relationship, we are also introduced to the characters of Justinus, Placebo, Damien, Pluto, and Proserpina.

Januarie

Januarie is so named because he is in the winter years of his life. The old knight has spent much of his life pursuing his "appetyte" for women, fulfilling his "bodily delyt" out of wedlock (lines 1249–50). He has finally begun to entertain ideas of marriage, however, and the advantages and disadvantages of matrimony are weighed in the tale's first stanzas. While bachelors are free in their pursuits, the narrator also categorizes them as full of "peyne and wo" because they have no security (1278). Instead, a man should settle down with a wife who can take care of him:

Ther as a wedded man in his estaat
Lyveth a lyf blisful and ordinaat
Under this yok of mariage ybounde.
Wel may his herte in joy and blisse habounde,
For who kan be so buxom as a wyf?
Who is so trewe, and eek so ententyf
To kepe hym, syk and hool, as is his make?
For wele or wo she wole hym nat forsake;
She nys nat wery hym to love and serve,
Though that he lye bedrede til he sterve. (1283–92)

This is what Januarie desires: a wife who will bring him joy and stay with him even when he's so old that he's bedridden. But even though he's already over 60, he's on the lookout for a much younger woman, as much to sate his lusts as to bear his children.

He wants "yong flessh" only, declaring that he won't settle for a woman over 20 years old; otherwise, Januarie won't want to sleep with her, he won't be able to sire a legal heir, and then he'll break God's commandment because he'll naturally be tempted to adultery (1418). Januarie sees no reason why such a young woman wouldn't be interested in him, however, and he believes himself to be virile as ever. Yet he also believes he's close enough to death that he needs to find a wife quickly, so he approaches his friends Justinus and Placebo for advice.

Justinus and Placebo

Januarie's friends are also aptly named: Justinus speaks justly, or truthfully, and Placebo speaks only to please; he is a flatterer whose words are ultimately empty. When Januarie asks Placebo's opinion on his plans to hastily take a young wife, Placebo repeatedly puffs Januarie up with praise and reassurance. He declares that Januarie is far wiser than him and that he must surely know best and not question himself. Placebo makes it clear, however, that this is how he coddles everyone above him in stature in order to stay in their good graces. His advice amounts to nothing more than that Januarie should do what he wants, and far be it from Placebo to suggest otherwise, which is exactly what Januarie wants to hear.

Justinus, on the other hand, advocates a more cautious approach to marriage. He believes such a serious and long-lasting enterprise deserves patient consideration, especially because marriage means you are trusting another person with your body and all of your possessions. Justinus advises that Januarie not pick just any pretty woman and instead try to get to know someone first to see if they would make a wise match. He also warns Januarie that, in his old age, he is unlikely to be able to satisfy a young wife. Januarie rejects the counsel of Justinus in favor of Placebo's approval, and he pursues May as his bride.

May

May's name likewise speaks to her function in the tale. She is in the fresh bloom of her life: a beautiful spring flower to be paired with the wrinkled and white-haired Januarie. Although she is poor and of "smal degree," Januarie finds May to be a suitable match because of "hir yowthe and hir beautee" (1625–26). Januarie is filled with desire for her, and he warns her that on his wedding night his passions might be too much for her. He sees himself as seductive and skilled in the art of love, but we are informed that May finds his efforts pitiable: "She preyseth nat his pleyyng worth a bene" (1854). A younger man also desires her, and May proves more impressed with his overtures.

Damian

One of Januarie's squires, Damyan, is described as "woful" because he "langwissheth for love" of May (1866–67). Presented as a traditional romantic hero pining for his lady love, Damyan writes dramatic poems about his feelings for May and carries them around with him. When one day Januarie sends his wife to visit a sick Damyan, he passes her a love note; although she tears it up after reading it, she is unable to stop thinking about the romantic sentiments expressed.

Whereas Januarie is described as pursuing his own pleasure with his wife, Damyan initially makes no move to seduce May and only wishes to share his feelings. May feels no desire for her husband, and she finds herself moved by Damyan's lovesickness. They exchange love letters but remain otherwise a loyal wife and a loyal servant, respectively, to Januarie—until chance arrives and Januarie is struck blind. Just as he has been, in effect, blind to the possibility that his young squire and his young wife might fall in love, they now can take advantage of Januarie's actual inability to see them committing adultery.

Pluto and Proserpina

The gods Pluto and Proserpina look down as May and Damyan engage in an affair right in front of Januarie, and they argue over the nature of men and women. Pluto wants to expose May's infidelity, and so he restores Januarie's sight so that he may observe the true nature of woman. Proserpina believes men are just as capable of disloyalty and often discredit women unjustly, so she gives May the gift of clever speech to convince Januarie he is mistaken in what he sees.

Although Januarie is once again able to see, he readily believes the explanation May concocts for why she and Damyan are so physically close. He would rather be blind to the truth and see only what he wants, as has been his character's nature from the start.