She has a strong personality, with very clear ideas about what and who she likes or dislikes.
As the princess of Ingary, where fairy-tales are true, there is a risk that her father will one day be obliged to offer her hand in marriage as a reward for some noble deed; but for the time being she is far too young. (She is a baby at the time of Howl's Moving Castle; she cannot yet walk or talk, but can pull herself up and 'babbles' - I'd guess she's about 10 or 11 months old. In Castle in the Air, she's a few years older.) Characters in both books attempt to claim her as a prize, only to retreat in confusion when they discover her age.