(spoilers in comment)
This was a very cute (albeit kind of sad) read and gosh I'd love to see more of this world because it was very intriguing and up my alley.
The monster creature was so cute though im kind of sad that allowing Fifteen to get close to it makes her and the thing disappear from Casper's life (I was hoping they could all get together and travel together at the end but alas,,, it was not meant to be in the endings ive seen gfhjgf)
The writing was so very good and the voice-acting really fit the characters too. I also like how you made the characters feel down to earth and fitting to their environment while also creating a sense of unease and uncertainty about this world (even to the point I felt unsure about the monster despite wanting it to be a companion because gosh it looks so cute but then it takes off with Fifteen and leaves Casper behind and...>0<)
Good job to everyone who created this i'd love to see more of this world if possible but at the same time I think this contained narrative works just as well :D