It’s not necessarily the thin waist that’s the problem. The whole build looks rather skinny and doesn’t give me the impression of Kylo Ren. TBH he would probably look a lot more like Ren if you gave him back his prefab chestplate.
To clarify, it’s not the worst MOC, and if you gave him a new mask and weapon I’d probably like him as much as the rest of your MOCs. If’s just that your usual style doesn’t suit Kylo that well.
You’re right that the thin waist is pretty much in proportion with everything else, but the head is just too big. I think a custom head would suit the build better. The build looks very posable though, that is a plus.
I feel like the torso is this moc’s biggest downfall. The arms and legs do a fantastic job of replicating the look and feel of his outfit in the movie, as does the head. The torso looks too slender and oddly textured in comparison. Also, the custom cape is way better than what LEGO gave us.
I’m going to be honest; he looks like a Chibi Kylo ren you would expect from a parody cartoon or something. Accurate, but his head is way too big for the body. The torso looks almost female in size compared to the head.
All in all I love the design but for the proportions to work you’d need to make your own head to scale.
I like it but the torso feels to Bionicle for a human character, but other than that and the things others have said about the head being too big, I quite like it