When you create a character for a story, be sure to think about their role in the story when you start, but don't get caught up trying to explain everything about them at the beginning of the story.
Let your reader discover new things about your character as they move through the book. Especially the antagonist. To maintain some suspense, tt's best to drop hints about them as the book unfolds, then reveal them when the time is right.
Unique characters with personality can bring life to your story. Pretend each character is like a friend you've just met. The longer you know them, the more you learn about them. Sometimes it's good. Sometimes it's not.