The war had been over for months when Mulan's mother gave birth. She was never a very brave woman and the thought that her sickly infant son should be made to fight if the war ever returned scared her. She was only human after all, and Mulan was weak. She feared for his safety. So she hid his true gender and named him a woman.
She knew her husband wouldn't understand; he was a proud man and the thought of hiding his heir from the honor of fighting for his country would be abhorrent to him, so she didn't tell him. It was easy enough to conceal the truth from him: he was never very interested in women's things, such as children he perceived to be girls, so while he was a kind and loving father he was absent from Mulan's life in ways that he wouldn't have been had he known the truth.
Mulan, to his own credit, knew better than to disobey his mother, and so he was the perfect daughter- if a little clumsy and adventurous where a girl shouldn't be. He befriended his father's horse and they had many long daydreaming sessions where he dreamed of life outside his father's compound. His grandmother was understanding of her daughter's plight and assisted where she could.
-o0o-
Mulan was beautiful, with pale skin and small feet (for a man, so they were still slightly large and clunky looking for his female persona) and a gift for smiling like an opening flower. Many a boy peeked over the Fa family's walls on a dare and fell in love with the "girl" within.
His eyes were almond-shaped, and his hips were wide, and his clothes were loose enough to give him a feminine, curving figure. His sickly nature led to him finding it difficult to put on much weight and in the end that only helped his disguise.
Life was good and Mulan dreamed but never dared leave the walls of his mother's imagined sanctuary from life, war, and death.
And then Mulan couldn't hide anymore.
-o0o-
The trip to the matchmaker's was a mistake; a test taken by Mulan's mother and failed miserably. The lucky cricket he had tucked into his pocket didn't help at all and he could only be grateful the matchmaker had not professed why he was a failure of a bride or he'd have eternally shamed the Fa family beyond repair.
That night, he ignored his mother's warnings and undressed before his father, showing off his flat chest and male genitals. He wasn't spoken to by his father for days. The day the letter arrived, Mulan had had enough. Trying to appease his father was going nowhere and he was sick of hiding behind his mother's skirts or his grandmother's tea cup or his own women's clothes.
He had no way of knowing his father was about to forgive him and turn his wrath elsewhere: Mulan's mother, who'd denied him a chance to even try to understand her reasoning.
Maybe if he had, he wouldn't have left as he did: dripping rainwater and in disgrace, wearing his father's armor and tying his shortened hair up in a tight knot above his head. He calmed Khan with his soft countenance and quiet words, and left for the army's camp, unaware he'd attracted two unwanted guests: the lucky cricket and his family's disgraced guardian, Mushu the dragon (travel-sized for convenience).
He believed himself to be alone in the world, and perhaps he was.
-o0o-
When his disappearance was discovered, his mother cried salty tears onto the ground his father knelt upon.
Fate didn't look kindly upon being tricked and it intended to make her pay for the insult.