I don't own Harry Potter or the number idea. That was an idea from a story I read. I just tweaked it to make it my own. (BTW, they are eighteen and Voldemort is dead)

Third person's POV

Dean had just gotten back from the holidays. He was exhausted and all he wanted was a shower, so he could go to sleep. But he soon found the bathroom occupied by a raven haired girl brushing her hair and dancing. "Um, hello?" Dean said, confused of why a girl was in the boys bathroom.
"Hi!" The girl said, her green eyes shining as she smiled. Dean blushed, a little understandable really. Her shining black hair, emerald eyes, and, admittedly, her body wasn't all that bad. "My name's Holly Potter. I'm Harry's twin sister. We met over the holiday." He could see Holly being Harry's twin, so he didn't question it.
"I'm Dean Thomas, one of Harry's roommates," He said, holding out his hand. She gripped it tightly and her sleeve fell back, revealing a tattoo on her wrist. "Cool! What's that tattoo mean?" Her hand shot back.
"What tattoo?" Holly asked, defensively a look of panic in her eyes. "Okay fine! I have a tattoo! But it's none of your business what it means." Then she hurried out of the room being careful to conceal her wrist. Later that night at dinner, Harry introduced Holly to everyone that didn't stay for Christmas and Dean and Seamus couldn't help but notice she gave an extra wide smile towards the Slytherin table, but Harry, Ron, and Hermione didn't notice. After being bombarded with a million questions, Holly excused herself and left quickly.
"Did you notice her tattoo?" Dean asked the table. Harry was suddenly very interested in the conversation.
"Yeah, I did. But as soon as I ask or get close to see it she freaks out. Even weirder is that she didn't have it before she came her. Did you see it?" He asked.
"I did actually. This is what it looks like," Dean answered and rummaged in his bag for paper and a quill, "This is it." He handed the paper to Harry and everyone gathered around it was ~50~.
"Let me see," Hermione said and grabbed the paper from Harry. After a few moments of inspection she smiled proudly, "It's an L!"
"An L?" Harry said incredulously, "How is a 50 an L?"
"I thought you might ask that," She said, smiling, "The roman numeral L stands for 50." He seemed to consider this for a moment.
"That makes sense," He said, "Her middle name is Lily, but why would she hide it?"
"May I make a suggestion?" Ron said, "I think it has something to do with a guy."
"A guy?" Harry and Hermione asked together.
"A Slytherin guy to be precise!" Seamus added dramatically. Harry looked at him as if he had gone mad.
"You're kidding, right?" He said, "Please tell me he's kidding!" A few people who had also seen her special smile in that direction gave him apologetic glances and pats.
"They must both have L's in their middle name," Hermione announced. Harry looked sick for a moment and looked over at the Slytherin table to find a familiar blonde head gone.
"Malfoy isn't here," He said, looking green, "And his middle name starts with L. He isn't here! Where is he?"
"Calm down, Harry!" Ron said, "It might just be a coincidence. Whoever she's with has the tattoo too so, just look at his wrists. Demand to see them unless he has something to hide." Suddenly, Harry looked so green and sick they were afraid he would throw up on the table.
"I just imagined that little… little ferret kissing my sister," He said, "And… and touching her. Touching! No one like him should touch her!" Harry stormed out of the room angrily. Dean, Seamus, Ron, and Hermione followed him out of the room desperate to convince him Malfoy and Holly disappearing at the same time was a coincidence. When they got back to the room, Holly was sitting on her bed in the Gryffindor girls' dormitory. She looked as perfect and glossy before she left. "Sorry I left so quickly," Holly said sincerely, giving Harry an apologetic smile.
"Oh don't lie!" Harry exploded, "I know you were with some Slytherin boy!"
"Um, yes, kind of. Actually, if you must know, I was feeling ill and Draco was so kind to bring me to my room," Holly said, "I'm sorry. I didn't know I had to ask to be alone with him."
"What is that supposed to mean?" He asked.
"Exactly what it sounds like," She shot back, "I didn't know you chose who I hang out with."
Harry's voice softened, "I'm sorry. I'm not in charge of you. I just don't want you to get hurt."
"I'll be okay," Holly said, "I know who I can trust. I'm going to bed. See you in the morning." With that she kissed him on the cheek and went to bed, pulling her curtains closed. After everyone was in bed a light flickered on from behind her curtains.

Hermione's POV

Everyone was asleep but me. I'm not tired really because I'm too excited about the L on Holly's wrist. Well at least I thought I was awake alone until Holly emerged from her bed. I thought about asking her if she was ill again. But then I thought better of it. I watched her leave the room and then, pulling a cloak over my pajamas, followed. I followed and followed until Holly stopped at the front of the Slytherin common room, said something in Parselmouth (A skill I assume she inherited as had Harry), and slipped inside. Knowing Harry would want to know about this, I ran back to Gryffindor and hurried to the boys' dormitory. "Harry, Harry!" I stage-whispered.
"What? What's wrong?" Harry answered, popping his head out the curtain with his wand brandished.
"Holly went into Slytherin!" I answered. Everyone else was starting to wake up.
"Holly's in Slytherin?"
"Is she sleepwalking?"
"Why didn't you follow her in?"
"Shush!" I said, "Don't wake up the other girls! They'll get cranky." Everyone was quiet.
"Tell us what happened," Harry said calmly but I could see the worry in his eyes.
"Okay, so I was awake. I couldn't sleep trying to figure out the L on her wrist. When suddenly, a light came on and she left the room. I was curious about where she was going. I followed Holly all the way to Slytherin. I couldn't speak Parselmouth so I couldn't go in to see who she was there to see. I knew you would want to know so I came here right away. That's it. That's all I know," I said all at once so by the end I was panting.
"I know who she was there to see," Harry said through clenched teeth, "Malfoy! I'm going to kill him!" He rose suddenly. I grabbed his arm and pulled him down.
"We don't know if he knows her as friend a let alone boyfriend," I said calmly, "Or if he has the tattoo." Harry went back to bed grumbling and once everyone calmed down and I was back in my room I hoped I hadn't given him false hope.