There are several characters in SNKL who began as a throwaway phrase in the source material. Yusuf came about because Mudarra needed a guide through Christian Spain, and Justa blossomed out of Doña Lambra having "a single maid" at one point in the story. They both took on their own amazing (in my opinion, because they seemed to develop independent of my original intent) story arcs.
Blanca Flor, however, is all mine. I took her name from other medieval epics and her personality from the question: What would Doña Lambra and Ruy Blásquez's daughter have been like? The epic states that they had no issue, but what if the case was more that after the terrible things their parents did, no one wanted to be associated with them, especially by blood ties? If I do write a sequel to SNKL, it will be based largely on that tension.
Personally, when I think of a female between 14 and 25 years old, I don't see a fully grown woman, but in the tenth century, people did in general. We're coming full circle with the way teenagers dress and make themselves up these days, but that's another story. So I took inspiration for Blanca Flor's look from her mother, seen in this post, and in this gorgeous artist's fantasy:
Then I made a sketch with words to evoke when I described her for the first time. Hard to see pencil in the scan, but here it is:
I was going for the beauty of the mother, but full of kindness.
Here's how the description ended up, from the point of view of Mudarra:
He ducked behind the tree
trunk, from where he observed a being who radiated so much brightness he hardly
dared to keep watching her, and yet he couldn’t look away. As he stared, he
distinguished two long braids the color of gold thread that pulled the hood
from her head and whipped from side to side and front to back as the girl-woman
changed her gait to suit her mood. Her mantle flew away from her body with each
step like the wings of a giant bird taking flight. At her neck, an underdress
of a fine, almost transparent fabric protected her fair skin from the blue wool
of her tunic. The skirt, covered in embroidered whorls, danced stiffly atop
soft leather boots. Mudarra thought he would visit a cobbler and have similar
boots made for himself in a strangely practical thought that ran somewhere
below the rapturous feelings the female caused in him. She distractedly passed
the empty bucket from one soft-looking mitten to the other as she made her
meandering way toward the riverbank. He had never laid eyes on anything like