Before you write a story, you need to know your characters. Really well. The more you know about them, the better your writing will be. You readers need to know more than your character's age, hair colour and clothing.
A great way of doing this is asking yourself a series of questions about your character, and then answering them!
For example, what's their favourite colour? If it's blue, this might suggest a calm character. If it's red, it could suggest someone quick-tempered.
What's their favourite hobby? If it's skydiving, you'll be subtly telling your readers they're adventurous. If it's train-spotting, you might be explaining they're perhaps a little less adventurous! You could use the questions from my character question and answer session page to guide you. (https://www.giannapollero.com/characters)
Give it a go - it's actually really good fun!