I think it's great that you're motivated to better develop your characters! One of the best pieces of advice that I have for character development (other than reading books on the topic, of which there are many), is to pay attention to people in real life and make note of the qualities that stick out to you. Whether it's a particular mannerism, or a fashion style, or a speech pattern, jot down those things and make lists of qualities that you can then go back to and pull from when you're writing, mixing and matching in ways that intrigue you.

In addition to posting this question in a static item in your port, you might want to consider positing it on the newsfeed and/or a message forum and/or a blog in order to draw more attention to the question and hopefully get more of a response.

I wish you the best of luck with your character development process!