In the books
In the A Song of Ice and Fire novels, Missandei is a slave native to the island of Naath, located off the northwestern coast of Sothoryos. She acts as a translator to Kraznys mo Nakloz, one of the Good Masters of Astapor. She possesses the typical Naathi features of dark skin, a flat face, and golden eyes.
While actress Nathalie Emmanuel was 23 years old when she was cast in the role, in the books Missandei is only 10 years old when she is introduced (though she is very intelligent beyond her years), younger than Sansa Stark but older than Arya Stark.
"Missandei" is pronounced "Miss-an-day".
- Missandei at A Wiki of Ice and Fire (MAJOR spoilers from the books)