In the first stage of any project, I do a ton a research a reference gathering. Not just on anatomy but material definition as well. Getting a feel for the materials themselves can help you make a decision on how to model something. Even though the concept was fairly cartoony and stylized, I knew from the beginning that I wanted to ground it more in realism yet still keep the flavor of the concept.
 

I wanted to try something that I hadn't done before so I built the character out of zspheres. It got me a quick basemesh that in the end I had to retopoligize to get the final details out of the highpoly that I wanted.

 

Here I started to rough in the form with the clay tubes, defining some of the facial features so I can start nailing some of his personality early on.

  To do the pants, I extracted the polygons from subdivision level 1 in max, cleaned it up a little bit, and capped off the ends. I then took this mesh into Zbrush to flesh out some of the major forms with the other armor pieces and straps in mind. I then kicked that mesh back into max to run a cloth simulation on it. The body, straps and armor were all set as collision objects for the simulation to get the folds to react and stress in the right spots. I had to meshsmooth it a couple of times to get some of the smaller folds and wrinkles in the mesh. This very quickly got me a great jump off point to detail out the pants.
  Another thing I had never experienced was polypainted so I figured I would give it a run on this project. Using the spray function in zbrush. I filled him with a very pale yellow gray and started painting on his vasular system. I then laid down layers of greens, reds, and blues to start building up his skin layer and color. I didn't get too much more detailed as I was going to leave the final bit of work on the skin and fur for photoshop. I rendered out a version of the polypainted highpoly that you can see here.
  To speed up some of my lowpoly workflow I decimated the highpoly down to a point low enough to be useable for the lowpoly and began stitching it to a low subdivision level. Granted, you have to clean up the geometry to have some good edgeflow, but I found this a lot quicker than starting from scratch.
  That same process was used for the head and in this particular case it works a lot better because it didn't really have to animate due to his mouth being tied down. Stitching together and cleaning up a bit got me a lowpoly within range of what I needed extremely fast.
Above are some of the maps that I baked out of max. On the left I have my poly paint information, normal map, and cavity matcap that I kicked out of Zbrush. In the middle I have my 3 different levels of ambient occlusion. I sometimes use 2 or 3 AO's to create a master AO. This gives me control over which areas get broad shadows and which areas I want really tight shadows. On the right are 3 masks I saved out of Zbrush from the highpoly to bake down to the lowpoly. I wanted these to help me paint the detail in the fur and muscles in photoshop.
In this above image I was testing out methods of presentation. Whenever building an asset for my portfolio I test the model out in different shaders and engines to see which one will give me the best look. They each had their specific feel to them. In the end I went with the Marmoset Toolbag.
  I knew that I wanted to pose the rat for his final presentation so I began building a bone structure that I would skin a quickly pose him. Never did I think I was going to create a full rig!
  I began adding functionality to the rig just because I got caught up in the moment. It is a pretty extensive rig including head parenting and an FK/IK arm switch. There were also some controls that were wired to roll the back, head and tail. I skin morphed in the end to get all the joints to bend just right.