Character Analysis
Michael is Howl's trusty apprentice, a nice fifteen-year-old kid who seems well-meaning and eager to learn. He's also secretly engaged to Sophie's youngest sister Martha, whom he visits regularly at the Market Chipping pastry shop where she works. Sophie approves of this match, since Michael seems like a steady young man with a good future ahead of him as a magician.
The thing about Michael is that he is frankly kind of blah. He performs useful plot functions, though. For instance, his trouble with the spell that turns out to be a poem leads him and Sophie to try and catch a falling star. Without that experience in mind, Sophie might not have been able to put together that Calcifer was once a falling star, too.
And Michael also gives Sophie his tragic backstory, which improves her opinion of Howl. He tells her that he was an orphaned boy in Porthaven who couldn't find a home anywhere, and that Howl was the only person in town who would take him in, since Michael had a reputation for bad luck. Howl's great kindness to Michael—and his somewhat stubborn refusal to say anything definite or concrete about that kindness—tells us a lot about Howl's true character at a point in the book (Chapter Nine) when most of his behavior still seems selfish and ridiculous.
Still, while Michael's presence makes all of these things possible, he doesn't leave much of an impression beyond sweet. We guess that between Howl and Sophie there's probably enough personality floating around the moving castle to make up for Michael's blandness.