Murakami is famous for his alternation between short stories and long form novels. The Wind-Up Bird starts with a short story that was previously published as "The Wind-up Bird and Tuesday's Women," although there are differences in the translation (he writes in Japanese). The premise of that short story alone is very similar to the effect of the novel broadly, so in form, it seems that the narrative structure of the entire novel is basically "variations on a theme" of the original short story, meaning that analytically, the novel is about a wife who isn't present in her marriage (she's almost not in the novel at all) and the unfortunate powers women have over Toru.
As a main character, Toru is complex. On one hand, he seems to have a cool, calm opinion of things, and he can content himself with a simple plate of spaghetti and a nice jazz album. But at the same time, he is insatiable. His best friend seems to be a girl who is too young for his advances. His wife is sexually unsatisfied by him. He doesn't do much in life. In many regards, he has been classified by his entire community as a weak man and a failure.
However, that's not the whole story; Toru also beats the tar out of two random men (who admittedly might be delusions). Therefore, a more elegant analysis of the novel might be that Murakami is designing a surrealistic story to show the adult effects of something like an Oedipal complex. Therefore, May, Kumiko, Nutmeg, and even Creta, are all symbolically the same character: They are variations on the theme of an "anima" or even a "succubus."