Haku is like Zabuza's little brother.
Haku will never know, because Zabuza must stick to his reputation.
Zabuza means the world to Haku.
Zabuza will never know, because Haku words it as, "I will be your tool."
They use each other. They use each other to be happy.
But no one is ever happy in a world of pain, despair, and hate. There can never be love, hope, or happiness.
Haku picks flowers, because it is the only way he knows of to show someone that they are loved. He gathers them in a bouquet of sweet-smelling rainbows of the earth.
He spends most of the day picking only the most perfect of each specimen for each breed of flower. When the end of the day comes, and he see's Zabuza's face, Haku's courage fails him.
Zabuza trains himself and his "tool" rigorously. He does this so that when danger comes, Haku will be safe.
He gives the boy bruises and broken bones, at first, black eyes and cuts. He hurts Haku during training, but it is for the boy's own safety.
Haku is Zabuza's little brother.
Zabuza is Haku's whole world.
And unto death, they shall not part.