Your setting will exist so that the character has somewhere to stand and it will help define them. You can use your setting to give clues about the character. For example, where they live gives you some hints about them - The squat vs the gated community. While the plot is more about the character discovering who they are, what their needs and wants are, in the process revealing to reader what they are really like.
All sounds so straight forward really doesn't it? Or is it? So often I see characters in work I am giving feedback on that all sound the same and are one dimensional mean that the reader just doesn't engage with the narrative.
There are many ways and exercises you can do to get inside your characters head. For example: write a letter to you the author from the character introducing themself; answer various questions such what is their favourite music, what's in their pocket, what's their deepest secret etc.
However, I saw a really simple way to think about characterisation when I watched the brilliant actor, Ian McKellen, being interviewed by Graham Norton (Check it out on iPlayer Graham Norton Show, Friday 22nd October) So simple in fact, it is obvious when you hear it, you wonder why you hadn't thought of it before. Sir Ian McKellen started off by pointing out how incredible it is that we are all different. Different faces, skin, hair, mannerisms, the way we walk, sit etc. He then went to show how he played two of his characters. Hamlet and his most recent one, Firs, in The Cherry Orchard. He did it purely by changing his stance in his chair. As simple as that. It was extraordinary. Hamlet was a young character where he sat up straight, while the other was an old man, so he stooped over. Just small details, but it makes a wealth of difference.
Become a people watcher. Create a notebook where you take note of people's mannerisms, hair, the way they walk. All those small details that you can use to paint a three-dimensional picture of your character with them. It's not about info-dumping, it's about hints. The notebook will be a resource you can use throughout all your writing, not just one manuscript. Keep it is as a live document that continues to grow every time you see something. Create those brilliant characterisations just like Sir Ian McKellen did through observation.
Conan Gray - People Watching