I just finished a re-read of 'Howl no Ugoku Shiro' and the characters are so vivid. Sophie is at the core—she's not your typical fantasy heroine. She starts off resigned to her fate, thinking she's plain and doomed to run her hat shop forever. The curse that ages her is brutal, but it's also what frees her from those self-imposed limits. Her internal voice is so sharp and funny, constantly complaining about Howl's mess or Calcifer's attitude while she's cleaning up the castle. That practicality masks a huge amount of courage, you know? She's the one who holds that weird, falling-apart family together.
Howl himself is a fantastic mess. All that vanity and drama about his hair and his reputation, running from his responsibilities—he's like a magical man-child. But you slowly see the cowardice has a real source, the deal he made with Calcifer and his fear of the Witch of the Waste. His relationship with Sophie transforms him, not because she 'fixes' him, but because she simply doesn't put up with his nonsense. They're both hiding behind curses in a way.
And you can't forget Calcifer! The fire demon provides the grumpy comedy relief, but his bond with Howl is the literal heart of the moving castle. Markl, Howl's young apprentice, adds this innocent, wonder-struck layer to the household. The Witch of the Waste is a great antagonist too, more tragic and complex than she first appears, a cautionary tale about what Howl could become if he let his power consume him. Michael, in the book, is a bit different—more central as Howl's actual apprentice—but the core dynamic of Sophie, Howl, and Calcifer finding a home in each other is what I keep coming back to.