All these characters belong to JKR, with the exception of the unnamed girl.
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Mr. Ollivander glanced up from his stack of wands to see Albus Dumbledore step from his fireplace and shake soot from his voluminous robes.
"Albus, you got my message," he said, clasping the man's hands in his own.
"Yes, Elijah. I apologize that I had not come sooner. I had a meeting to attend to." Dumledore vibrant blue eyes smiled. Ollivander knew he had been organizing wizards and witches against Voldemort. "You said you had something important for me to see?"
"Rather, a someone," he said, cleaning his spectacles on his robe.
"Oh? Who is it?"
"A young woman who stumbled in here earlier today. She seemed rather distraught. It seems she was trying to find this person." He took a crumpled piece of paper from his pocket and handed it to his old friend.
Albus smoothed the paper, and there, in hurried but clear lettering, was the name of his Potions Master: Severus Snape.
"She was looking for Professor Snape? Did she say who she is?"
He shook his head. "No, no she did not. I'm afraid she collapsed soon after and seems to recall… nothing. I have her in the room in back." He led the Hogwart's Headmaster through to the back of his store where he kept a spare bedroom for long nights making wands, but stopped short just outside the door. "Albus, when she gave me that sheet of paper, she did say one other word."
"What word?"
"Betrayal." Ollivander whispered it, knowing that single word was very powerful when used in connection with Snape's. It could have many meanings. Deadly meanings.
"Never a good word when used for anybody," Dumbledore said, seeming to read Ollivander's thought.
Ollivander nodded in agreement, then pushed the door open. The woman, no more than twenty-five, sat on the bed, her back against the wall, with her knees drawn up to her chest. She raised her head when the two men entered the room. Two bright silver eyes stared out at them from behind the long black hair, which flowed over her otherwise pale face.
"Do you know who I am?" Dumbledore asked politely. She shook her head. "Can you tell me who you are?" Again, she shook her head. "Do you know who you are?" His voice became gentle as he sat on the bed near her and slowly pulled her hair from her face. She shook her head again, her eyes never leaving his. Dumbledore simply looked back at her. She seemed familiar. He knew he had seen her before, but could not place her. Odd, very odd, he thought. "My name is Albus Dumbledore. This is Elijah Ollivander. Do you remember Mr. Ollivander?"
"Yes." Her voice was soft, musical. "He helped me."
Dumbledore nodded. "Yes, he did. You said you were looking for someone." He showed her the piece of paper from Ollivander. "Do you know who Severus Snape is?"
"No, I do not."
"Why are you looking for him?"
"I do not know." Large tears were brimming on her eyes
Dumbledore exchanged a look of desperation with Ollivander. "When you came in here, you said 'Betrayal.' Do you know what you meant?"
"I do not, sir." The bell in the store rang, signaling a customer. The girl jumped and gripped the blankets of the bed tightly, as Ollivander hurried out of the room.
"You're all right. No one here will hurt you. We're trying to help. Now, do you remember anything at all? How you got here? Where you were before? Who your parents are?"
"I was afraid."
"Of what?"
"I don't know, but I was very afraid. That's all I remember."
Dumbledore sat back in quiet contemplation. If he alerted the Ministry, she would be sent to St Mungo's. But what did this girl know? What is the betrayal, and how does it relate to Severus? Dumbledore knew he would have to take her back to Hogwarts. It's the only place she would be safe, and where he could watch her constantly.
"Child," he said quietly. "I am the headmaster at a school. If you like, you can come back there with me. I will give you a place to stay and we can, perhaps, attempt to return your memory. Would you like to come back there with me?"
She hesitated a moment, deciding whether or not she trusted this old bearded man beside her. Finally, she nodded consent.
Dumbledore trusted the girl into the hands of Madame Pomfrey to be checked for injuries, then called Minerva McGonagall to his office to explain the situation.
"Do you think it is prudent, having the girl here?" Minerva asked when he told her the story. "We don't know anything about her. She may be dangerous."
"Right now, she is simply scared. She doesn't know anything about herself. We must help her, if not for herself, then for Severus. Has he returned yet?"
"No, he has not."
Dumbledore sat back in his chair, deeply worried. Snape had been summoned to Voldemort the evening before, as he often had been during the summer. But it was rare for him to not return by morning.
"She spoke of betrayal. We need to know whether she knows of his betrayal of Voldemort, or if it is he who has been betrayed."
"Albus, has it occurred to you that Snape could betray you?" the deputy headmistress asked gently.
"It has. But by Merlin's will, I hope it not be so."
McGonagall quietly agreed, then asked, "So what do we do with her now that she is here. We can't have her wandering about the school while the students are here."
"I agree, we cannot. We will give her a room on the third floor, with explicit instructions not to leave that floor. The students must be warned not to wander there."
"I believe we gave those instructions once before. They were unheeded."
Dumbledore agreed. It was only four years ago that Mr. Potter, Mr. Weasley, and Miss Granger found their ways there, even upon warning of death from the Headmaster, to find the Sorcerer's Stone.
"We will have to keep the curious in Gryffindor under control this year." Minerva winced. She was the head of the Gryffindor House, and they were her responsibility, a rather draining job between Harry and his friends and Fred and George Weasley, a pair of identical twins who loved to cause trouble whenever they could. "If they must serve detentions every evening for the entire year, they will, but they will not disobey. They must stay out of that corridor. If Harry is found anywhere near that corridor, I will deal with him myself. Make sure he is aware of this."
"I'll see to it, Headmaster."
Dumbledore glanced up at Minerva's use of his formal title in private. "It is for his own good, Minerva. As you said, we don't know anything about this girl." He sighed. "I still have many friends at the Ministry. I will see if any of them can give me any information about her. Perhaps we can learn, at least, what her name is."