I've been rewatching Code Lyoko on Netflx Instant, and this just occured to me. She's about the right age, so that would surely come as a huge shock to her.
WARNING: Not for guys who can't handle hearing about periods.
Aelita's chest gently rose up and down as she slept, dreaming of the world she had seen so little of. Dreaming was such a new and unfamiliar thing to the girl. Stranger still, sleeping was a new concept as well. She had only been free from Lyoko for a week after all. Something had began to disturb her sleep though. There was a warm, sticky feeling at her inner thighs. At first, Aelita was only vaguely aware of it, but it soon woke her up.
"What is this stuff?" The young teen poked at the red and brown stuff coating her skin and the sheets. Once she woke up a little more, she realized that it was blood. While she knew about blood, this was the first time she had seen it herself. The biggest thing she knew about the stuff itself is that it indicated that one was injured.
"A-Am I dying?" Aelita started to panic, stumbling out of bed. "Maybe it's a side effect of the materialization? I don't remember getting hurt..." She snuck out of her room and tiptoed down the hallway, her heart beating faster and faster as she then ran as quietly as she could over to the boy's dorms.
"Jeremy!" The girl loudly whispered while knocking on the door of her friend's room.
"Hahhh?" Jeremy yawned and reached out for his glasses next to his bed. "Just a second..." He got up and headed towards the door.
"Jeremy, something's very, very wrong with me." Aelita told him through the wood, hoping that would hurry him up. He could hear the panic in her voice, so he woke himself up and opened the door, ushering her in, knowing they'd be in big trouble if someone caught them in the hallway.
"What seems to be the problem?" Jeremy closed the door and straightened his glasses, then noticing the blood on her hands. "Wah! Aelita, that looks like blood! What happened?" His face whitened a little at the sight, then grabbing some tissues and wiping the blood off.
"That's the problem. I don't know. I just woke up, and there was blood everywhere. Do you think it's a X.A.N.A. attack? Or a problem with my materialization?" She told him, praying that he'd know what was wrong and how to fix it.
"Well, where's it coming from?" Jeremy sat down at his computer, checking for any activated towers.
"The waste removal area." The newly real girl answered, thinking back to when she was first materialized, when Jeremy and Yumi had explained how to 'remove waste'.
"The wa-" Being as smart as he was, it didn't take very long for him to realize the problem. However, now there were new problems to deal with. "Erm..." He stuttered, turning pinker by the second. Out of everything he knew, this was probably the subject he knew the least about. The logical thing would be to take her to the infirmary. The nurse would be able to both help and explain. Unfortunately, it would be incredibly suspicious to have a girl know absolutely nothing about it. There was no doubt that that would raise red flags.
"Well, uh... You don't need to panic." 'Einstein' assured her.
"I don't?" She sighed with relief. "Why not?"
"Uhhh... Because it's completely normal. Hahahaha..." The young male chuckled nervously and picked up his phone, speed dialing Yumi, feeling a tad guilty for disturbing her.
"Yeah...?" The Japanese girl sat up and rubbed her eyes, figuring she needed to bounce into action and get to the Factory.
"Sorry, but it's an emergency." He told her as he got some baby wipes out of a drawer, then handing them to Aelita.
"X.A.N.A.?" She asked him while standing up and throwing on some clothes.
"No, actually. Aelita has... started her menstrual cycle." Jeremy turned red as he went to face the door, giving his friend some privacy while she cleaned herself up.
"Ah? Oh. I'll be right over."
"So that's what the pains were earlier today..." The pink-haired Lyoko Warrior was happy that there was a good reason.
"They're awful, aren't they." Yumi half sighed, half smiled.
"Actually, I like them. It means that I'm alive." Aelita had the warmest smile on her face. "I'm truly alive."