I begin tucking him into bed and he tells me, "Daddy check for monsters under my bed." I look underneath for his amusement and see him, another him, under the bed, staring back at me quivering and whispering, "Daddy there's somebody on my bed." -

What is happening here? Who are the characters? What are their backstories (tell me about their personal history) ?

What happened next?

The characters in the story are: Daddy, the child, and the other child, who is hidden under the bed. What is happening here is that we, the readers, are not sure about who is the real child and who is the ghost, because we don't know if ghosts see humans as monsters or not. We are not sure if ghosts are afraid of us or not. What we clearly see is that both children are afraid of the other, and the father is the one who is in the middle of both. As regards the backstories of the characters, we have in one side the father, who is suposedly scared due to the spiritual nature of one of the two children in the room.

According to the description of both characters, we can assume that the ghost is the one who is under the bed, but he is also shown as a fearful character, so we can infer that both characters are afraid of each other.

About the ending of the story, probably one of the children disappears and the two remaining characters will be scared about it.