I REALLY like the style you're developing. That first painting is an instant favourite, but I think you should make the highlights more defined and brighter on that robot, since it's metal and reflective/shiny. Also the lighting and shading on the robot's upper left and right legs is off compared to the rest of the picture.
I like the Koyoma sketch. Nice execution of perspective, but his right hand is way too big compared to his head and his forearm way too short. Clothing folds feel misleading and random on that arm as well.
Lizza is great, except for the bumpy shadows and highlights. Make longer more confident strokes that have a solid form.
Sir Damien shows good understanding of general form and facial anatomy, but the light source isn't very clear due to the fact that some parts of his face on the left are too dark, when his hair is darker on the opposite side. Also when drawing metals try to make your darks really dark and your lights really light, and have them separated clearly to some extent. The armour here looks really dull-ish when it should shine some more. Also softer blending would be nice, but that could be just my preference.
Thanks for the awesome feedback Salt! It really helps. I have a bad habit of being timid when it comes to making dark darks on metals, I always have this fear of overdoing in and instead tend to "underdo". I can understand liking softer shading too, especially on metal, I've just recently started to take a different approach to painting and blending purely with the paintbrush, not with a water or blending brush. I'm not completely sure if I like it yet (it took a hell of a lot longer to do than just using a blending brush) but I felt that it gave it more of a painted effect, since I was trying to go for a medieval style portrait. Either way, all of your observations were spot on, thank you very much! (And I love your signature, SexualLobster is one of my favorite animators)