I did a quick demo the other day to my school org about using shapes in designing characters. I cleaned these designs up for the sake of this (small) tutorial.
Essentially, I chose certain shapes (in this case, trapezoids and ovals) and distributed them throughout the designs. I used this way of design thinking in the designing of a group of characters.
Other than just using similar color schemes, props, and uniforms, I believe using this tactic helps solidify the designs of a group of characters even more, making them more consistent to each other.
Doing this exercise is also a way of identifying your shape language and visual style.