(I'm super unoriginal in this please don't hate me. I just don't want to make entirely new characters every time I write something)

(Also this is all basically a black butler reference)  

Prompt: "So that's what you are," he whispered in awe.  

Something has always followed him. It lurks in the darkness, watching his every move. Nobody knows it's there, or what it does. He just knows: it's there. The young boy has tried to talk to it a few times, in his childish, friendly-like manner, but got no reply. But the boy knows the creature in the shadows cares, though, because the kids who had bullied him had gone missing a few days ago, and everyone was nice to him now. So the boy just assumed that the shadows were his guardian angel.

"Vince!" His mother called from downstairs, startling the boy from staring to the darkest corner of the room.

"Coming!" Vincent called back, scrambling off of his bed and onto the solid floor of the Lothar manner.

Once, and only once, the boy had left the room and ran down the stairs, the demon stepped out of the corner of the room. He chuckled to himself, the boy possessed such a pure soul, such curiosity and pureness of the heart only came from children Vincent's age. Unaware of the horrid events of the world around it.

Vincent had tried to talk to him again, and the demon couldn't help but sigh. It was annoying, to be frank. He didn't want to converse with the kid, just be there the exact moment he dies to harvest his soul before the reapers got to it. Alas, the life of a demon can be tough, especially when you don't want to make a contract with other disgusting souls. The sound of Vincent running back up the stairs snapped the demon out of this thoughts. He sank back into the shadows as Vincent burst into the room, grabbing his coat off of his bed and running back to the door. Before he left, however, the boy stopped and looked at the corner once more.

"I think I'll call you Sebastian. It was the name of my dog."

And before Sebastian knew it, the door was being slammed shut. Sebastian blinked, stepping out once again and assuming his physical form. The name of the child's late dog...? How undignifying. No matter. The demon stole a glance at his left hand, grimacing at the mark positioned right in the center. The boy just formed a contract without even knowing... No matter. This just guarantees the collection of the boy's soul at the time of his death.

Sebastian made his way out of the room, changing his form to match the needs of the boy. A personal butler shall suffice.

~~~

Years later it had come time for the boy to die, the contract mark placed carefully on the center of his collarbone fading away until it was barely visible. Sebastian kneeled down, peering into the boy's eyes with his crimson gaze.

"Tell me one thing," Vincent wheezed, moving one of his hands from the bullet wound in his abdomen to touch Sebastian's cheek, to have reassurance that he wasn't alone. Vincent's frail fingers left a few lines of blood as they slipped down to the floor. "Was it you...? The creature that had followed me when I was young?"

Sebastian closed his eyes, looking down to the ground. "Yes, my lord. It was me."

"So that's what you are," he whispered in awe.

(please don't hate me-)