Note. I am not the original creator of Portal 2. All rights go to their respective owners.
Process & Inspiration
I started by taking a look at how the character of "P-body" from Portal 2 was constructed. He is a long and slender robot, with several mechanical apparatures in between his joints.

I wanted to simplify his design for the sake of making the animation more minimalistic.
First i drew up his joints and connected them, then started working on a more oval
looking bodyform. I also decided to make the eye the centerpiece of his body.

After that I added details like the different black lines, eye ring, and orange button.
Below you can see a comparision between his original design, and my simplifed version.
Character Rig
After experimenting with different combinations of joint-movements, I settled on one that makes it possible to convey more expressions. As his eye never changes, the joints have to be able to express more emotion.