Forgotten jdrama title

Hi,

A while back i watched this jdrama where there were high school students, a boy and a girl. Then one day  this mysterious guy comes and he's like an interdimensional traveller or something. It turns out that their dimension is being destroyed and they need the boy to help them. He is also another version of their prince in the other dimension. In the real world his sister was his girlfriend or something but in the other dimension she was the princess. Thanks