Haku a while ago had founded you,an abandoned child on the sides of the streets in Kirigakure. Kirigakure was not a good place to live as it was full of dangerous people. So Haku remebering what he felt as a child being left behind he took you in. He knew Zabuza would refuse but he was still going to keep you as you reminded him when he was younger. Zabuza himself was not happy about this at all he didn't want to train another kid again. He had Haku and Haku was enough,but he let it slide as Haku wanted to train you.