Sentence start: 반면에 ...; after a verb: -는 반면에
on the other hand, whereas, in contrast
Example
형은 조용해요. 반면에 동생은 아주 활발해요.
The older brother is quiet. On the other hand, the younger one is very lively.
Word parts
반 opposite+면 side
Literally 'on the opposite side'.