Ling Mei - I agree with you in that she's still too plain. I recommend adding some kind of light headgear/accessories to emphasize that this isn't a Leixia 3P.
Dark Knight - This might just be my preference, but his armor is too ornate. If you really want to stick with those pieces, I'd recommend giving them a more monochrome color scheme. A little bit of detail color is fine, but not too much. I like what you did with the bat wings on the cape, though.
Succubus - I can understand why you still go towards a bare design on this one. She's a succubus, it's part of what she does. Try out some of the chest armor and belts and see how far that takes the design.
Shoko - Despite it being a relatively simple design, it actually works. I'm not sure what part you used for her headwrap, but I like it. I would have appreciated seeing some modification on the part colors, as a lot of these parts are still stock-colored, but red, black, and gold work well together, here. It's one of the few cases where I've seen a red and black color scheme not look generically grimdark and that's refreshing.