What's the difference between flat and round characters?
A flat character will have a fixed set of (usually predictable) traits. None of these are likely to drive or create any real conflicts within the story.
Round characters are more complex and have traits that often conflict with one another.
An old man can be both a miser with his time and affection, but generous financially to the charities he supports.
The mother can be frivolous with her money, but responsible when it comes to her career or children.
The teenager may not speak to his parents, but will willingly offer his friends advice/help whenever they ask.
The best characters are rife with contradictions.
There's nothing inherently wrong with incorporating a flat character into your story, but they don't deserve center stage or even a strong supporting role.
Keep them strictly to the background.
Be Brilliant!
~Katie~
