This story is quite creepy and insightful. Although it is an internal monologu, which can becme a bit boring, you kept interesting by allowing Riku some small interaction with Kairi's lifeless body. I would have enjoyed just a bit more interaction with the outside world however. It only heightens internal monologue in my opinion. I imagine this taking place in Neverland and I don't think a little bit of external detail and setting would go amiss.
This story is an effective study in mental dysfunction. Riku's intense jealousy is given more than once source. He's jealous of Sora's new friends. He's jealous of Kairi. He's jealous of Sora's new skills. He is emotionally torn in so many directions that this becomes a very effective portrait of madness. I started to catalgoue all of the different fears and motivations which you explore in this short story, but I found myself writing far too long a list. It's amazing how you pack all of that into so few words.
Riku's fixation on the Shadow Sora is obviously the nice touch that brings it all together.