The Pieces of the Past

My name is Kazuki Nakarima, now going to stay my first day of work in the ads company. Well it's a story about how my life goes with the someone I used to know back in the past.
What really happen back then. After those years? How you feel when you meet someone who uses to called them your friend? The one person who I thought would leave me behind? Who I love and now suddenly she appears back in your life? Not know what reason? Is she fooling me or am I really a fool to begin with.
Yumi Sachiko, what am I to you?