The best tool is a character profile questionnaire.
Start with the basic information such as age, eyes & hair colour, body build, her clothing style, her voice.
Here, Alexandrina wears a sophisticated gown, even if she is a young girl.
In a previous page, I showed how little she cares about her clothing but wanted to please her mother.
Alexandrina's features are slick with a strong jaw. Her eyes are pitch-black and her skin is dark.
As I have no means to show colour in my manga pages, I will insist on rhetoric as for her red, shiny hair.
Alexandrina's body is lean like a bamboo stick. She has long fingers and a tall neck, enhanced here by a low-angle shot.
Her beauty doesn't outweigh her intelligence - she is clearly thinking here about what is the best course of action for the letters she is holding.
The other elements you will need to add to the questionnaire can be quite diversified. You can expand to the following elements:
Morals & ethics
These are elements you need to consider carefully as they will get in the way of the character's daily behaviour towards her family and friends. If she despises everything and everyone, she won't have friends, and her family will barely talk to her. If she's extremely strict, she won't accept any mistakes from anyone, or even herself.
Education & finances
The fact that your main character's family is rich or not could play a major role in your plot, regarding your themes and the development of the storyline. You can however easily dismiss it if wealth is not one of your major themes in your manga. Education can be a strong element to use in your storyline if you want to develop your characters in a private or public school, an element you can find quite often in shojou mangas. That could develop into a strong story between your characters, but careful not to end up with too many clichés.
Identity vs. Persona
...is the element you could use when you state the qualities from your character's point of view, what she think of herself (Persona) and the qualities from the others' point of views (Identity). It's important that you write down all you can in that category as it's going to help you lay out motivations and fears from all your characters.
Interests, Motivations & Fears
Building up your character's hobbies is really important as it could gear towards her future career and studies. What interests her might lead to a strong set of motivated thoughts or end up with occasional fears of failure.