Youth Has Passed, No Longer Human 1

Just who is whose substitute? She casts aside her dignity and is willing to give up everything to pursue him. He curses her, despises her, and even treats her as a backup, yet she still greets him with a smile. Caught in love's tangled web, he gradually falls deeper, but who is the black-clad boy who's been following them all along? Why does his appearance change her heart? And how does she become the moonlight he can never reach for the rest of his life?