The below description is taken from LEGO.com.
“Why, yes, I am indeed a walking, talking skeleton.”
There the Skeleton Guy was, out trick-or-treating one dark Halloween night, when he suddenly found himself in a land of real-life witches and ghosts and vampires. He doesn’t dare to take off his costume and reveal that he’s just a regular person underneath, so he’s doing his best to pretend to be a monster too, and hoping that he won’t be discovered.
But what the Skeleton Guy doesn’t realize is that he isn’t actually fooling anyone. Everybody knows that he isn’t really a monster (after all, you can’t fool a werewolf’s nose), but they also don’t care. He seems to be a nice fellow, if a little strange, so as long as he keeps being a friendly neighbor, the other monsters think that he can dress up however he likes!