She is the most beautiful of the three sisters, and considered it highly unfair that, as is well-known in Ingary, only the third of three sisters will find a great fortune.
When Mr. Hatter died and the girls had to be taken out of school and apprenticed to trades, Lettie hoped for something that would let her go on learning. Instead, Lettie is sent to be an apprentice at Cesari's Bakery in Market Chipping, where, though she might be destined never to come to much, she has a chance of meeting a handsome young apprentice and living happily ever after. Lettie and Martha are unhappy with their arranged apprenticeships and using a spell Martha finds, they swap places.
She makes a small appearance in Castle in the Air.
See also: Howl's Moving Castle (spoilers)