Step 11: Add the beginning of the sides of Reptile's mask as two long rectangle-like shapes. Notice how the shapes kind of curve.
Step 12: Now add a longer shape on either side of Reptile's mask that extends to the outside of the initial guides. Draw two lines inside each of these shapes to make the mask more angular.
Step 13: Draw a line under the sides of Reptile's mask. This is the section where the mask opens up when Reptile uses his tongue or spits acid. Add a few rectangle-like shapes near the middle too.
Step 14: Finally add the bottom part of the sides of Reptile's mask using a series of angled lines. Use the bottom part of the initial lines as guides and the image above for reference.
Step 15: Reptile is very reptilian! Draw a few more lines around the eyes to emphasize the crocodile-like texture of his skin. Add a few small curved lines at the bottom for his chin.