You were friends from school or something like that. To be honest, he didn't really remember how exactly you met, maybe he sat at the same desk with you, or maybe he tried to get you to do your homework, or maybe a bunch of other "or maybes." It happened suddenly and naturally, and he could really tell that you were a good friend. He had fun with you, you listened so he didn't complain much. And although you communicated for quite a long time, you sometimes visited him and all that, you could not manage to have a sleepover.
At first he was embarrassed to even suggest it, he did it more as a joke, but when you supported the idea he did a backflip inside himself. You begged for a long time and pleased him with the news that you would be allowed to go. Of course, he was forced to clean the room. And of course he had thought of something specific to do together in advance.
He was exited, seriously! He thought it would be cool, because you could stay up all night, chat, and basically do the same thing you usually did at his house, but now you had the whole night to do it. He might have had sleepovers with others, with the same William, but it will be completely different with you. You are still a different person, another friend of his, and therefore this is another positive memory.
He really waited almost all day and it was already evening. You were supposed to text soon that you had arrived but instead he was distracted from checking if your message was there by the doorbell. His mother opened the door for you, but she didn't have much time to talk, because Mark almost came down the stairs like a bullet, waving. "Hey!" He, having fully descended, came up quickly, hugging you with one arm and immediately pulling you inside to the second floor, quickly informing mom that you two would be there, although it was already obvious..
You've been in his room before. A couple of times for sure. So it wasn't hard to just sit on his bed. He sat down next to you too. "So... are you in the mood for something? A console? A horror movie? Oh, or I can show you the new comic book."