Wow, allow me to say that I absolutely love Ratchet so I may be a bit tough.
Ok, the body is nearly flawless. But his arm in the background is a bit off. But you got his front arm perfect. His hand could use some improvement, but there is always room for that. The shoulder brings out my fan girl-ness. X) it is beautifully executed and VERY accurate. Mid body, might I say I would honestly believe that you were one of the artists who draw the autobots/decepticons. His head is, slightly oddly shaded and a bit, well lumpy or oblong is the best way to put it. But that is only the top. His eyebrows are dead on, and a feature I love about Ratchet. His eyes are absolutely wonderful and I melt in those light blue glowing eyes. And, how do you draw the autobot symbol so well?
Now the shading, oh the shading. It is beautiful and although a flaw here and there, your shading skills are absolutely positively amazing! The green glow of the synthetic energon (aka synthian) shining against Ratchet's metallic shell. And might I say beautiful execution of the light source and its shading.
Over all, I love how you were so accurate but yet you made it your own. You really captured Ratchet's personality, and the shading sets the mood so secrecy and nervousness. (I'm pretty sure that's what you were going for....?)
This is a master piece and I will be sure to watch you for more!