Title: 3/10
I have no idea how this title relates to the story…  I don’t even remember it being said in the story.  I actually really like it as a title.  But not for this story.  I’m sure you have an explanation for how it relates, but if, as the reader, I can’t make the connection, you need to make it more obvious.  I actually thought the Chapter 3 title, My Greatest Sin, would have made a good title for the overall story.

Also, for chapter titles, I don't know for sure what the rules are, but I would suggest capitalizing them.  Endless Forest, My Light, My Greatest Sin, A Pair of White...

Plot: 30/35

                3/5: Originality
                The whole light versus dark/ demons falling in love with angels thing isn’t really original.  But I still enjoyed the way you spun the story.  The ending was much more original, with the twists you added.  But you need originality up front to attract the reader.  Something in the forewords/description or the first chapter that tells your readers that your story isn’t like the others.

                9/10: Believability
                I thought you did a good job with the believability in this story.  You took the time to explain things and make it believable.  I talk about this more in narration, but I think you could have made this even more believable by ‘telling’ less and ‘showing’ more.

Side note: If Kyungsoo knew Jongin was going to be beaten and tortured in the Tower why didn’t he fight harder to keep his father from taking him there?  I thought he was just going to be locked up.  When I found out he was tortured and Kyungsoo knew he was going to be tortured, I couldn’t believe Kyungsoo didn’t fight harder or reason with his father for a better alternative.

                10/10: Narration
                This story has a fairy-tale type feel to it.  “Once upon a time, a demon was born…”  But I really liked that you told it in First Person point of view from Kyungsoo’s perspective.  That made his feelings for both Kai and his father stand out more.  Part of the reason this feels like a fairy-tale is because you ‘tell’ the story more than ‘show’ it.

I lived here and did this.  And then when I was 17 this happened.  I felt this and I thought this.  That’s ‘telling.’  ‘Showing’ involves more descriptions and more explanations.    Here are two websites that explain this in more detail:

http://emmadarwin.typepad.com/thisitchofwriting/showing-and-telling-the-basics.html

http://www.writersdigest.com/editor-blogs/there-are-no-rules/showing-vs-telling-in-your-writing

I actually think this story is just fine the way it is.  I like the fairy-tale feel and it adds character to the story.  But this ‘show vs tell’ aspect is very important to know and can improve any story.

                8/10: Setting
                Again, the fairy-tale type ‘telling’ doesn’t need a whole lot of setting.  You did a good job always ‘telling’ us where the characters were in the story.  But that doesn’t help us visualize the setting.  I really liked your description of the Tower.  You describe the atmosphere, the sounds, the smells.  You don’t actually describe what the Tower looks like (and you should), but I still feel like I’m there.  Every scene s