This little girl looks so much like me when I was a child & how my cousin's daughter looks now. I am extremely fair like this child.My hair was redder than this child's (more strawberry blonde, but had I been in the sun a lot, or probably would have been a lot lighter, like this child's). Also, my eyes are blue-green - they look green in some lights and blue in others. Interesting to note the difference of opinion on this girl's eyes.
My dad's father was from Ruthenia/Galicia - not Spain but the region between Ukraine & Poland, in the Carpathian Mountains. My dad's mom was Swedish. (My mom is a Florida Cracker- yes, it's a real group, just as Wikipedia.) This girl seriously looks like she could be a member of my dad's side of the family. I think this child is probably of light-skinned Slavic descent (Estonia, Poland, parts of Ukraine, Russian -particularly maybe of Finno-Ugric descent, Belarus, perhaps Scandinavia)...yeah, I know that doesn't really narrow it down much.
As for why this little girl didn't get darker in the sun, those of us who are very very fair skinned CANNOT tan. Like at all. I burn in a tanning booth for 5 minutes. I can't "build up" a tan. Even with prolonged sun exposure, I just burn and get freckles. No tan. My dermatologist even told me someone with my heritage needs to avoid prolonged exposure to the sun between 10-2. This child is so fair that I doubt she can tan and get darker. But prolonged exposure surely should have given her some freckles (even with high number sunscreen I often don't burn but do freckle). My skin on my feet is just slightly darker than the straps on my flip-flops, so I guess I can technically get a tan - just not one anyone else would notice or call a tan. This child must have been well-covered (hat maybe? And long sleeves?) or perhaps she stood in doorways or in shade or something. Even as a little kid I remember that other fair-skinned had freckles because their moms and dads allowed them outside in the sun more than I was allowed because of my pediatrician's warnings to my mom, which were the same as my dermatologists. You can definitely get freckles/skin damage at a young age. Since they were using her since she was blonde and fair-skinned, perhaps they were also bleaching her freckles? (yes, it's a real thing- just google "bleaching freckles" - women have been doing it for ages in places where fair skinned is or was prized- there are home remedies that have been used for centuries and newer chemical/dermatological ones).
I know at times who I've been in places where such fair skin and hair is really rare, people have been super interested in me. In southern Spain, where most people are dark (largely due to the Moorish control of southern Spain from 711-1492) women would ask to touch my skin and hair. I had random kids follow me and ask the same thing. My fair-skinned family member also had this happen in India. I went to a party as a teen at a middle eastern friend's home (where I was the only non-middle eastern person) and the other girls asked to play with and touch my hair and they asked me if I bleached my skin (I was surprised to learn this was common practice among some of them and that they tried to stay out of the sun to avoid getting darker). The girl who does my pedicures is from Vietnam and is always telling me she loves my skin when she does my toes. She told me in Vietnam light skin is prized because it it's indicative that you don't work outside - you're an upper class "lady" rather than a peasant. The crazy former dictator of Lybia, Gaddafi, had a harem of fair-skinned Eastern-European nurses (mostly from Poland, Russia, & Ukraine-
http://www.huffingtonpost.com/2011/02/28/galyna-kolotnytska-gadhaf_n_829093.html).
Just trying to add some insight into why this family would want a fair skinned child, especially a very fair-skinned child. I haven't been to Greece, but I'd assume such a fair child would be an exotic possession, so to speak, and worth well for begging & unfortunately, this exoticism would probably help her earn a lot of she was sold as a wife later on or put to work as a prostitute. I am so glad she's been rescued. I'd gladly raise her (heck, she even looks like she is related to me).
Sorry for rambling....