So this was a really popular animation by mike intel made during 2015 so yeah here's my theory

[There should be a GIF or video here. Update the app now to see it.]

So probably this is what I thought about the story

The girl and the boy at the start were playing and going trick or treating yes obvious and the mother took a picture of them, then at the last halloween you could see when the boy walked back home with bruises and '#' those weren't makeup but somehow he got bullied. When the boy sat with the girl together you could see closely that the right leg of the girl have somehow trap marks but a bunch of people noticed that but some didn't. When the boy was running around getting chased by his friend, he ran into the bullies that bullied him which explains the bruises and '#' on the boy. When the boy introduced the girl to the bullies, the bullies didn't care cause they couldn't see her and thought that the boy is just imaginating things so they walked away, when they did, the girl got mad and wanted to fuck them up to show how kind she is. Once they were trick or treating, you could see that the boy got a lot of candies while the girl got non it's because the girl was invisible cause she's a ghost, we'll get to that later. At the part where they ate the candy at the start of Last Halloween you could see that the girl's note says "You Found Me." even at the part when they were near the fence you could see the same note saying "You Found Me." on the girl's hand and it meant "Goodbye" but the boy didn't understand so he ignored it. The part where you could see the boy's mother and some random man with a Doctor, and when the boy took the notes and stuff it said "Diagnose: Imaginary Friend" the mother thinks that her son in imaginating an imaginary friend while the girl is real, when the boy was telling them that the girl is right next to them, you could see the girl wasn't there, and when the mother showed the picture that she took, the girl wasn't there which explains that the girl is like a ghost or something. When they were at the fence, the girl wants the boy to follow her into the forest and lead him somewhere, the boy didn't want to but he doesn't want to stay alone and stuff so he followed her. You could see that the part where the boy was running trying to catch up to the girl, you could see he avoided several bear traps and he avoided the last bear trap also, it might because the girl have some supernatural powers and stuffs to keep the boy safe from the bear traps. Once they got to the tree, they ate their candy, the boy's note says "I Found You." and the girl's note says "You Found Me." the exact same notes from last time and the reason it's like that because the girl was dead and she came back as a soul because before the last halloween, she promised the boy that she will meet him again at the last halloween, but instead she died and she wanted to keep the promise so she came back as a soul. At the end you could see the decayed body of the girl with insects and maggots eating her, and you could see when the polices flashed the lights to the boy he was alive, and when they shined it to the bear trap it explained the whole thing why she had a trap mark on her leg, when the female police hold up a book and searched the page where the missing girl was it said "Last Seen: At home." she probably ran away from home after arguing with her parents and ran to the forest, she wandered around the forest and she got lost, she was trying to find the way back home instead, she stepped on a bear trap making her start to bleed to death, so she ran to a nearby tree and ate her last candy to survive, but she knew she will just die there from lack of blood so she had no hope but to wait for people to find her or just stay there and die, when she did, her body started to decay and insects and maggots starts to eat her body, the boy didn't get eaten by any insects and stuffs which may tell that the girl had died like months ago and nobody searched for her, but when the boy went missing, the cops knew where he was but not the girl, when they found the boy he lead the cops to the girl too meaning that the note said "You Found Me." and he found the girl which tells the note saying "I Found You." and the soul kinda disappeared after their mission being succesful for bringing the boy to the real body of the girl which everyone is able to see. The body of the girl was there the whole time and the girl wanted to lead the boy to it. And the part where the girl looks at the boy and look at the fence she was thinking if she should just lead him to the dead body of hers to complete her life or just let him go, so she decided to make him follower her to the body. The girl promised the boy and she wanted to stay with the boy one last time before finishing the mission and let the soul rest.

That's what I thought of what the story meant :/ man I'm just getting matpat shit up in here