Before diving into character design or backstory, I think it would be helpful to first think about the role these characters will play in the story. Are they there to support the main characters? Are they there to provide a contrast to the main characters? Once you have a clear idea of their purpose, it will be easier to create their design and backstory.
For the character design, consider their physical appearance, clothing, and any unique characteristics that will help them stand out. For their backstory, think about their motivations, their relationships with other characters, and any significant events in their past that have shaped who they are. Thinking about what purpose these background characters have in relation to your main characters will help a lot with these answers!
Overall, the key is to make sure these characters are well-developed and have their own unique personalities and stories, even if they are only in the background. Good luck!