Character design is very important. As key element of your story they will add or take away from the comic. Your audience should like or at least have a connection with your heroes and supporting cast while either understanding or truly hate your villains. For comics, character designs also include costume design and powers. Here's a simple guide on how to work the process.
Concept
This is the rough draft of your character. Whether a hero or villain, you need to build them up and give them purpose. Here's where you come up with the back story, name, powers, and motivation. Make them interesting by getting away from stereotypes and the back stories. Use things you know or stuff you research:
" Madeline, lost her sister in the war and blames Unicorp, a big war supporter and weapon's manufacturing corporation. Madeline was trained to be the best infiltrator in the war and now after her sister's death she has gone rogue and infiltrates Unicorp for their true purpose in the war and to expose them to the world."
Get yourself a database of names. many writers have a book of names from different cultures and nationalities. Keep cultural websites book marked or keep reference material handy, especially if you plan to have authentic diversity in your comics. You also want to have a list of super heroes and villain names that you might want to work with. Try to accommodate the name to the super power or even function of your character Choosing powers for your characters can be both awesome and frustrating. The main thing here is not to over do it. The more powers, the harder and more complex the challenges you have to create to keep your comic interesting. Today's reader is constantly asking why or how come...you need to provide an intelligent and reasonable answer. A quick search online will provide a list of common comic book super powers.
Costume design is important. Choose a design that builds or complements your character's role or power.