It's not very good, but it deals with Oscar learning at 13 that she was not a boy after all. It does mention in the manga that she believed she was a boy until she was thirteen.
So, uh, here it is, pre-revision, because I need to go to work.
If it seems a little... mean... it's because I've just finished Oniisama he. Damn, that anime was MEAN.
Oscar Francois de Jarjayes sat in his room, scrubbing his breeches so hard his knuckles were nearly white. No one could know what happened that day, especially Nanna. The last time Andre accidentally hurt him while they were sparring, Nanna cracked a wooden spoon over his head. The resulting bruise didn’t fade for two weeks.
To be honest, Oscar wasn’t quite sure exactly what happened. The two were off in the gardens, practicing their feints. Oscar felt a dull, throbbing pain in his abdomen. It was distracting enough that Andre scored hit after hit. He didn’t strike roughly, or so Oscar thought- upon returning to his room, Oscar noted the dull reddish-brown stains in his underwear that had soaked through to his breeches. No matter what, it wouldn’t come out. Where exactly did Andre hit him, anyway? Surely not there.
The next morning, Oscar woke up and noticed an even larger stain on his sheets. He was still bleeding. This was obviously not a good sign. He began to panic. If the bleeding didn’t stop, he could die! He dressed, making sure to stuff his undergarments with a rag to soak up the blood, and dashed outside, wrapping the soiled undergarments in his sheet. He barged into the stables, startling Andre, who was brushing the horses.
“Andre, are any of the mounts ready?” he panted.
“Well, I prepared yours first… Oscar, are you all right?” Andre asked, concern etched on his face.
“I’m fine, Andre, just fine!” He hopped up on his mount and took off, tearing through the Jarjayes grounds. After about twenty or so minutes, he hopped off, digging a hole under a tree with his bare hands and depositing the gruesome bundle underneath it. It had begun to smell terrible. It wasn’t the coppery smell of blood that wafted from the bandages the last time Andre accidentally injured him. This was far worse. He was relieved when it was finally underneath the earth.
Oscar made a point to avoid everyone for the rest of the day. He didn’t want any questions or any sympathy. He wasn’t entirely sure what was wrong. There was no wound, though by this point his thighs were smeared with this vile substance. He sat on his bed, wondering who to tell and how. He didn’t even understand it himself.
A knock sounded at the door. Before he could respond, his mother glided in gracefully, softly shutting the door behind her. She sat on the bed beside Oscar, taking his hand gently.
“Dearest, there is something you and I need to speak about.”
Oscar’s eyes filled with tears, though he blinked them back. Young noblemen should not cry. Madame Jarjayes blinked softly and touched his cheek. “Is something wrong?”
“No, Mother. Nothing is wrong.”
Madame Jarjayes paused to consider her child. Oscar had just turned thirteen and was growing to be an attractive, kind, and noble young adult. It was time to tell her.
“Oscar, I am not entirely sure how to tell you this. It is indeed a strange situation. You see, you are not like Andre, and you are not like your father. Despite what we have told you all this time, you are not a little boy.”
“What do you mean, Mother?” Oscar looked lost.
“What I am trying to say is… you are not a boy. You are a girl. You were born a girl, yet raised a boy. Your father needed and wanted an heir, an heir I could not give him. So, he named you Oscar, and we vowed to raise you like one would raise a son.”
“Mother… you’re lying to me. Stop it.” The tears again. She had to fight harder to keep them back.
“Have I ever lied… oh.” Madame Jarjayes took a deep breath. “Oscar, listen. One of these days, you are going to begin to change from a little girl into a woman.”
“I’m not a little girl, Mother!”
Madame Jarjayes continued softly, as if Oscar never interjected. “You will get taller. You will grow hair under your arms and between your legs. And, once a month, you will begin to bleed. That’s how you’ll know you’re a woman, Oscar, because you’ll bleed every month. It’s God’s sign to us that we are ready to have children, that we are fertile, mature women.”
Ready to have children? What nonsense was this?
“Your chest will ache as your breasts begin to grow.”
Breasts? How will I properly fence with melons on my chest, like Mother has?
“These changes are changes all women go through.”
“But mother, I don’t want to be a woman! I’m not a woman! I’m a boy! Oscar is not a woman’s name! It is not!”
Madame Jarjayes merely held her daughter closer. She didn’t know what to say. The speech that had excited all her other daughters filled this one with fear. She loved Oscar just as much as she loved all her other children. But, for the first time, Oscar seemed a stranger to her as she jerked back, standing shakily. She turned, staggering away from her mother, slamming the door to the next room behind her.
She’d come to terms with reality, one day. And she’d realize that Madame Jarjayes was only doing what she thought was best for her daughter.
Madame Jarjayes hoped so, with all her heart.