Armagedon

Hundreds of millions of years ago, highly-advanced beings looking to make a planet for themselves purposefully tampered with Earth's natural processes in order to create multi-cellular life. Watching the planet's evolution closely, they continued to help life evolve further in the hopes of having actual civilization develop.
It's now 1996 in Seoul, where Hyesong, a high-school student, struggles with the usual problems of youth. The only unusual thing about him is that he's been having the same apocalyptic nightmare for an entire month; in the dream he is an great fighter and genius tactician who faces off against a seemingly-unbeatable foe.
Are these dreams not really dreams? Does he really have a role to play in the cosmic balance of the universe? And is the new girl in his class really just a normal student?

From back of book:

He-Sung has dreams of the Earth being destroyed by alien invaders! In these dreams he is a motorcycle-riding warrior, the last defense against the alien armada. By his side is a beautiful armor-clad girl with strange powers and futuristic weaponry. But these are just dreams, and they don't mean anything... right? That's what He-Sung thought... until his mystery dream girl appeared as a new student in his school! But if she is real... can the aliens be far behind?