My guess is that after the narrator sent his balloon out in the school project, a crazy guy who had been a stalker in the past happened to get it. He then began hunting the poor kid down, finding out everything he could about him, while taking dozens of pictures of him. The narrator soon befriended Josh, who, except for the color of his hair, looked very similar to the narrator. After a while, the narrator started thinking there were footsteps in his room, and thought he was sleepwalking, as he was waking up in various places. (Josh later mentioned this had been happening to him.) Eventually, he woke up outside, lost in the woods, while his mother found a runaway note on his bed. Apparently, the stalker intended to kidnap him that night, and left him alone a while for some reason. But, his plans were ruined when the narrator actually got back home safe.
They later decided to move (In the story Boxes) and ended up moving faster than planned when his mom saw what was under the house. The stalker had apparently lived under there, and kept trying to lure the cat under there so the boy would come in by himself to catch it, in which the stalker would have him. However, they would always find a way to catch Boxes without having to go under there, so that plan didn't work. After they moved, the stalker had trouble finding their new home, so lived in the old one, hoping to lure them back. After the narrator and Josh came back to get Boxes, the narrator found what was under the house, and the stalker met Josh and took his picture, apparently intending to mess with him later. (Which, unfortunately, is what happened) They ended up getting away, but left the walkie-talkie there, and had not been able to get Boxes. Later, the stalker found the walkie-talkie, and let Boxes' meows come through, just to mess with the narrator.
Later, him and Josh decided to explore the woods there and make a map of them. There also lived an old lady (Mrs. McGee) nearby, who always invited them in. On that last day, when they heard the stalker following their raft from the woods, the stalker apparently went in to the old lady's house. Upon returning, the narrator discovered that the stalker had messed with his clothes. When he was about to leave, the old lady showed up and started talking to him, but didn't invite him because the stalker was there, and was apparently trying to protect him in her own way. After she went back home, the stalker killed her and chopped her into pieces, and then left.
In Screens, the boy was now 15, and had lost contact with Josh. He had a crush on Josh's sister, and from her discovered that Josh had left a runaway note, just like the stalker left on his pillow. (Making it evident the stalker had actually gotten Josh.) Now the next part is kind of up to speculation, because this is where Veronica dies. My guess is that the stalker was a pedophile who was sickly in love with the narrator, and got jealous when the narrator got a crush on Veronica, and so he tried to kill her. Before leaving, he took her picture, and then later, stole her phone; tricking the narrator that he was Veronica while texting him. He then met up with him at the theater, after texting the narrator, "I love you". Later, the narrator found out Veronica had been dead the whole time, meaning the stalker had sent all the messages.
In "Friends", you discover that the stalker had apparently settled on getting Josh since he looked similar to the narrator, and had kidnapped him and done who knows what with him. (Meaning the stalker was also the one who left the note.) They later found Josh's body, laying in a large coffin with the stalker, who was now dead too. It mentions that this guy also looked in love and serene, while Josh's hair had been dyed to the same color of the narrator's, and given the narrator's old clothes to wear. (From his old house, that the stalker had occupied. This is why they were too small.) So, basically, the sicko had kidnapped Josh, forced his "love" on the poor kid, and dressed him to look like his original target (the narrator) and had gotten Josh's oblivious dad to bury them in the crypt/coffin thing so they could have their final moments together. (ugh) Also, it looked as though Josh had managed to kill the guy in the crypt, and had to sit there and slowly die of lack of air and nutrients, with the stalker's dead body with him the whole time. So, basically, he had a horrible death.

But, fortunately, the stalker was dead too, so the narrator never had to worry about him again....