는 is a particle in Korean. They're a little complicated to explain, but basically, it means that whatever comes before 는 is the subject of the sentence. In your sentence, it basically has the meaning of 'what about (Mika)?' or even just 'And you?' So if you asked them 'Do you live in Korea?' they'll answer 'Yes' and then 'What about you, Mika?' Using 는 is just a very simple way of directing the question back at the other person.