The Reason (5xDrabble)

It had been months ever since Zabuza had picked Haku up and decided he could use the younger Shinobi. Mainly the swordsman thought Haku's kekkei genkai would be a handy tool to wield, and once the kid would grow up under his tutelage he could mold Haku into the perfect tool to fit his grip – purely loyal to him and obedient to a fault.

However things started to become different when Zabuza uncovered Haku's secret…

They were on the road somewhere in the area around Kirigakure, seeking a new hideout as to not be caught by Kirigakure ANBU. Fortunately the mist was hiding their tracks and Zabuza urged his companion on as he ran through the swamp. Suddenly he heard a dull thud and a splash. Expecting an ambush he turned around, hand on his beheading sword. But there were no assailants to be seen. Instead Haku had dropped to the ground, seemingly unconscious. With a disgruntled noise Zabuza picked the frail body up. He wouldn't want to lose his tool so soon. As he carried Haku he noticed how hot the youth felt, and he assumed the kid ran a fever. Cursing his luck Zabuza looked for a makeshift shelter where he could treat Haku, and he found one in an abandoned outlook. As he applied cold compresses to decrease Haku's fever he noticed the kid lacked something all men must have. A bit flustered Zabuza redressed Haku and put on even more layers of clothes, telling himself a whitewash that Haku needed to sweat the fever out. The older one didn't touch the youth again after that, or at least he kept bodily contact to a minimum.

When Haku awoke feeling slightly better, Zabuza grumbled: “You should have told me beforehand. But never mind, I'll still keep you, boy or girl. But from now on you can't tell anybody you're a girl. You must become strong, stronger than anybody. And for that you have to pretend you are a boy. Because I won't have a weak partner, got me?”

Haku nodded eagerly, for she certainly didn't want to be abandoned by Zabuza. Zabuza still treated his partner as a tool from then on, maybe even moreso as he now knew she was in fact a Kunoichi. Still drilling it into Haku that she had to pose as a boy might have protected the kid more than once. Zabuza was no fool. Men would instinctively back off his tool if they thought Haku was a boy.

And so, years later, when they were still roaming the Land of Waves, Haku confidently told Naruto she was a boy – much to the Genin's shock. Haku had absorbed Zabuza's words as a dry sponge soaked up a droplet of water and ingrained it into her mind and heart that she was a Shinobi. And maybe she even believed she could only be useful to Zabuza as a boy.

But we might never know, as they died together, pretending until the very end...