Remember: a character is defined what they do and say, not by their physical appearance. Therefore, use physical descriptions to bring out their key traits. First ask yourself who this character is and what important details of their life and personality you wish to convey.
As ever, the apps I have reviewed here are not necessarily apps that I particularly liked or disliked, but are simply a random selection of writers’ apps that I have tried out at one point or another. As usual, these reviews only reflect my own personal opinions and impressions, squished, squashed and squeezed into a few short sentences.
A foil is a character (or sometimes an object or idea) who highlights the traits of another character (usually the protagonist) by contrasting with them. But apart from that, these characters can play just about any role in your story you like.