Missandei was originally a slave who served as an interpreter to the masters of Astapor, until she was freed when Daenerys Targaryen took the Unsullied army and used it to overthrow the slavers. Missandei now serves Daenerys as her trusted advisor and Handmaiden. After Daenerys flees Meereen on the back of her dragon Drogon, Missandei remains in the city as part of her Small Council along with Tyrion Lannister and Grey Worm while Jorah Mormont and Daario Naharis search for the missing queen.
When Daenerys returns to Meereen, with a horde of Dothraki, she achieves peace by ending the war over the Liberation of Slaver's Bay, which is later renamed the Bay of Dragons. Missandei then sails with Daenerys in order to reclaim the Iron Throne.
(source GOT wiki)