"Who is Sofia?"
Translation:Kiu estas Sofia?
Kie is easy because it means "where." Physical location only, it never means "whereas."
English "what" can correspond to "kio" "kiu" "kia." The most common correspondences are who-kiu and what-kio, but they are not direct translations.
"Kio" asks a question that expects an -o-word answer: what category of thing, what kind of person. This answer is usually not definite, but that's not a hard rule. - Kio estas Sofia? - Sofia estas trajna konduktoro. -- What is Sofia? - Sofia is a train conductor.
"Kiu" asks a question that expects a choice or identification (which is why it's more common with people). The answer is usually definite with tiu or la. - Mi ne jam patronis ĉi tiun restoracion. Kiujn vi mendas kutime? -- I haven't been to this restaurant. What do you usually order?
"Kia" asks a question that expects a description, usually with an -a-word, kiel-phrase, or prepositional phrase. - Mi pensas pri eliro manĝi. Kian volus? -- I was thinking of going out to eat. What (kind of food) would you like?