Tower of the Sun by Tomihiko Morimi

ABANDON THE PURSUIT OF ORDINARY HAPPINESS! One young man’s barren college life changes forever when he shares a budding romance with a girl named Mizuo...only for it to all come crashing down when she has the gall to dump him! With the specter of a solitary Christmas Eve looming, he tears through the streets of Kyoto with just his powerful (some might say delusional) imagination to protect him from the cruel world at large. A Japanese fantasy novel for every man who’s been let down by love—and those still bracing for it!