I think to write a character well you really need to know them and I don't just mean what they look like. Below is list of questions I ask myself before I start writing about a character

1. Strengths

2. Weaknesses

3. Personality

4. Are they a good person, bad or somewhere in between?

5. What drives them? (Ex: Are they driven by power, love or to protect their friends?)

6. What event in their past shaped them?

7. What are they striving for in the future?

8. Do I want them to be liked or hated by my readers?

9. One word to describe them?