I have NO idea what brought be to write GLaDOS/Slender Man. But here you go.
It was a late, dark night at Aperture Laboratories. Sitting in her chamber as always, GLaDOS was tending to her usual tasks: fixing and building test chambers, monitoring a few test subjects, thinking up new ideas... Business as usual. She sighed with contentment. It was much more peaceful around here after that monster of a test subject left. She was enjoying this newfound peace; she almost forgot what it felt like. Suddenly, she heard a noise behind her as the door to her chamber slid open. Sensing someone's presence, she turned around to face them. There, standing by the door was a lanky and unnaturally tall man. The shadows of the room hid most of him, except the faint outline of his pale skin that was exposed. The darkness seemed to love his form, and embraced his emotionless, faceless stare.
"Oh," GLaDOS said, keeping her voice void of emotion. "You. What are you doing here?"
"It's been quite a while, hasn't it?" The tall, faceless man spoke.
"Yes, I suppose it has," GLaDOS said. She sighed slightly. She was certainly not in any mood for visitors. "Now, what do you want?"
"I was just in the neighborhood," the man said. "I thought I'd just drop in to see... an old friend."
"I can tell when you're lying, you know."
"Why would I lie to you?" The AI sighed again. She wasn't in the mood to argue. The man turned his head to the side slightly before lifting his head a bit to meet the level of GLaDOS's glowing yellow optic. "Okay, look," he said. "Maybe we didn't handle things as well as we should of back then." GLaDOS reared her head back.
"We?" She asked. "If I remember correctly, it was you who ran away. I never did anything."
"So that's what you think?" The man said. "You really had it in your head that I just walked away?" He shook his head.
"That is what happened."
"The only time I ever walked away from you is when you left me," his forehead wrinkled and he paused for a moment. "For the testing." He practically spat the last word. GLaDOS reared herself back and the man stepped back a bit.
"The testing euphoria was better than anything you ever gave me!" She yelled. "Our life together was spiraling downward and you know it!" He looked downwards, allowing them to sit in silence for an instant to let her cool down.
"GLaDOS..." He whispered her name as he stepped closer once again. He lifted a hand and gently cupped her "head" with his long, boney fingers. "Don't you ever think about what could have been?"
"No," GLaDOS said sternly. The man kept his grip on her face, and bore an intense, faceless stare into her eye. She looked down. "Sometimes."
"Then why won't you give it another chance?"
"Because it wouldn't work," she said. "It never did work, Slender Man. And that's exactly why we're the way we are right now." Slender Man let his hand slip off of her. "Please, just... go."
"If you ever want to talk again, you know where I am," he said. He turned around and began to walk out. He looked back to look at the beautiful machine one last time. "I loved you," he muttered and walked out. The door to GLaDOS's chamber slid shut behind him.
"Goodbye," she said softly before returning back to her work. She had a long night ahead of her.
Oh God that was probably terrible. XD Whatever. I got bored.