The characters of The Wife of Martin Guerre can be relevant to the modern reader of the 21st century because they are, actually, quite universal: they have weaknesses, limitations, and "demons to conquer". They also break the rules, make mistakes, and do not follow a set standard of behavior as it would be expected. This being said, any reader, at any time in history, could appreciate the characters of The Wife of Martin Guerre.
Take Martin, for example: rather than following the expected roles imposed upon him as the elder son of a feudal family, he breaks with the social norm, escapes the family, abandons his wife, and disappears. How is that for a modern-day "player", or "deadbeat", like the modern reader would say?
Then we have Bertrande, who is the ultimate submissive wife...but then reveals a sort of a she-devil side when she actually falls for the intruder that tries to pass for her husband. What is actually interesting is that we know that she "kind of suspects" that this is an intruder. However, here is her husband's own family telling her that she has to accept the intruder and even share the bed with him. Could we not agree that this would make Bertrand "the one who had the last laugh?" Indeed, she is!
Arnaud de Tilh is a likable character, because he represents the rogue and ultimate romancer. We know that Bertrande finds something quite alluring about him, which makes her quickly accept the family's request to take him in as a husband. Moreover, we know that he must have much more tenderness and sensuality to have Bertrande's love at the end. His ending comes as a sad consequence for his lies, and we do sort of suffer along with Bertrande for his loss. Furthermore, we also feel the weight she carries in having to also respond to Martin, who despises her without having even asked for forgiveness for having left her.
Hence, the characters are quite realistic and share similar emotions as any other person in the modern world. We all have experienced heartbreak, allure, sensuality, the need to escape, the need to do "something" mischievous, and the curiosity to break the rules of norm. The characters in The Wife of Martin Guerre cathartic take us through those processes, and open themselves up for us to analyze their weaknesses, and compare them to our very own.