I love the pen-tooling great work. I do though don't like the brush glow you did that creates minor orbs from the origin of the glow.
Makes it look blurred imo.
This piece lacks depth as well.
Spend some time on adding more elements and making them look good to take up some space this will help with the depth part.
Or if your in a hurry make your canvas smaller.
I highly suggest looking at tutorials on depth or just playing with it really!
The renders seems a bit sharpened.
Like on his head and other smaller areas.
The lighting is really off, and there is no light source.
There is a glare on his helmet, but where's the light?
You can manipulate any render to have any lighting you want on it, it's quite easy. and you can make it have shadows for depth as well.
Along w/ the render you need to do the same w/ the rest of the piece this will create more natural lighting and flow.
I can not stress how much lighting, shadows, etc will effect your piece. If you spend the time to make sure it all looks right some mad depth can be created.
You have some great concepts here though I like the style!
Overall due to depth and lighting
Somewhere around 6.8-7/10