The Abandoned Sonic Restaurant

Ever heard of the restaurant company called Sonic? No, not a restaurant based off of Sonic the Hedgehog, The Sonic Drive-In Restaurant. Originally, there used to be 2 in my town. But, one closed down because of the manager's recent debt, and him being evicted out of his house in the town. He had to move in with his parents in Massachusetts. There's been gossip around mostly seniors in my university that he was evicted because he was charged with Necrophilia, Murder, and pretty much every crime in the book. I was skeptical at first, but then I learned of the abandoned restaurant's location.
It was 7:00 PM, and I was told to never go there before 7:00 PM at all costs, and that i'd have to leave before sunrise. The man who told me this was a close friend, so I trusted him and drove to the abandoned restaurant. When I finally parked the car, I heard sounds that sounded like children being brutally tortured. Oh yeah, I forgot to mention that I was told to never go downstairs, he said that he comes there at night to torture local children.
I saw a desk with a letter on it. It said: "''Nobody must ever find out my real name. I've been hiding it for 50 years. The last thing they should know is my real name. I'm moving out of this borough to go back to [Untranslatable Text, and Scribbles]. Not even my close friends know my true name, and even my father has forgotten. I should just leave this restaurant to rot. The new Sonic Restaurant is making me lose all of my customers. Plus, i've been in debt and even this job can't pay it off. This will be my final letter I ever write in this cursed borough. Someone recently broke in and has been living downstairs. Whoever he is, it is NOT me. Or whoever IT is. I don't even think it is a human. Well, if someones reading this in the future, I advise for you to never look behind you. He's always watching in here. Remember that.''
-Ezekiel
Ezekiel isn't the managers real name? I wonder what it is. I texted my friend about "The manager isn't downstairs. Who where you adressing?" He replied with "...I never said it was the manager, Vincent." I gasped. I instantly thought that whatever or whoever was downstairs was a demon, devil or some other sort of infernal spirit. I slipped, but I wasn't on the floor.
I fell on the body of an unknown individual. I can't look behind myself. I'm tempted to look behind though. I ignored it and found some half eaten burgers that must've been a decade old. Some half eaten french fries, stuff I expected from here. But then I found a knife. On the floor. I grabbed it and ran out of there as soon as humanely possible. When I went in my car, I saw 2 red eyes staring at me, and he seemed to be waving. I made an oath to never go there again. But one question hasn't been answered. Who was that thing downstairs?
I texted my friend about who it was and he told me everything he knew about the matter. "It's a demon that haunts the basement. You know all the crimes the manager was accused of? The manager never did it. The reason why he changed his real name is because it means corpse. He's never been in the town since the day after he saw that demon with his own two eyes."
And then a skeleton popped out.