Step 11: Draw a couple of curved lines over each eye for Dr. Eggman's furrowed brow. Robotnik doesn't have hair on his brow. Angle the lines toward the nose for a menacing look.
Step 12: Darken the initial circles to create Dr. Eggman's glasses. His glasses clip to his nose, so they don't have earpieces. Draw a few diagonal lines within the circles to represent reflection.
Step 13: Draw a line under Dr. Eggman's mustache for the shape of his mouth. Notice how the bottom corners of the mouth touch the edge of the original circle.
Step 14: Inside the shape of the mouth, draw a long, horizontal line to represent where Dr. Eggman's teeth come together. Draw a few smaller vertical lines along the horizontal line to indicate the individual teeth. Shade in a small section on the sides of the mouth to indicate where the teeth end.
Step 15: Use the initial circle as a guide to complete the rest of Dr. Eggman's head. Darken the top part of the circle except for the section behind the brow on the right. Make the bottom part of the head flatter and more parallel to the shape of the mouth.