Howl’s Moving Castle became popular not only with its enticing story, but with the ever-so-attractive male protagonist, Howl Jenkins Pendragon.
In the film, it was mentioned that Howl has a reputation for making women fall in love with him, yet breaking their hearts in return. Look no further than the Wicked Witch of the Waste, who grew jealous of Sophie because she got Howl’s attention. This led to Sophie being cursed to be an old woman.
Interestingly, fans are curious whether or not Howl has ever been in love with Lettie. Lettie is Sophie’s younger sister. Unlike Sophie though, Lettie is vibrant, bright, and friendly. She also noticeably wears colorful dresses and accessories in comparison to Sophie. She is also very popular among men, evident when they were trying to vie for her attention when she is working at the bakery.

Since Howl is known in town to date a lot of beautiful girls, then it might be possible he knew Lettie. But what is the exact nature of their relationship? The Studio Ghibli film and the original book differed in that aspect.
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Film: Lettie Knows Howl, But Never Met Him
When Sophie visited Lettie, she told him about her encounter with a wizard. Immediately, Lettie assumed that it could be Howl. But she told Sophie to be wary of him, because Howl would steal women’s hearts and eat it up. This is not in the literal sense, it only meant that Howl simply left a lot of women heartbroken.
Judging from Lettie’s reaction and warning, we can assume that she has never met Howl. Howl never tried to approach nor date her. She only knows Howl from the widespread rumors and stories she has been hearing about him. They have no interactions at all in the film.

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Book: Howl Seemed Interested in Lettie
Howl and Lettie actually interacted in the Howl’s Moving Castle book by Diana Wynne Jones. Lettie has more story in the book. She works at the bakery, but she actually switched places with her and Sophie’s other sister, Martha. Martha is studying magic from Mrs. Fairfax, which does not interest her much. Lettie, on the other hand, wanted to learn more about magic – which led to them switching places. They also managed to switch their respective appearances too.
Howl was at first taken in with “Lettie” in the bakery, in which it is actually Martha. We can assume that Lettie’s beautiful appearance has caught his attention.
Sure enough, when Howl managed to encounter the real Lettie later on, he tried to court her. However, Lettie did not seem too interested in him. Mrs. Fairfax validates this that Lettie is more fond of her other suitor. However, Sophie assumed that Howl is in love with her sister.

It can be interpreted that Howl is actually courting Lettie for the purpose of knowing more about Sophie. This is how he knew about Sophie’s curse. It may be that Howl is starting to have feelings for Sophie at that point, but tries to avoid it.
Howl’s entire thing with Lettie is only brief. After Lettie, Howl was taken in by another woman’s beauty – Miss Angorian. But this too seemed only fleeting, because at the end of the day, he still chose Sophie. He also seemed intent on protecting and rescue Sophie from danger without even fixing his appearance – a contrast to his vain personality.
So to answer the question of whether or not Howl is in love with Lettie – nope. He may have shown interest at first because of her beauty, but his true feelings are with Sophie. Instead of courting Lettie because he is genuinely invested in her, he uses this to know more about Sophie instead.
It can also be said that Howl’s past flings and relationships may not even be deep. It was only mere infatuation from his part, but he does not have a sense of commitment there. His interest dies immediately, hence breaking a lot of women’s hearts. It is with Sophie did he truly feel love, metaphorically representing how Sophie was the one who returned his heart.
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