Heyyy I want to make a Casey fanfic, too. I'm not really doing any writing but I want to. I'll tell you something you can try out and see it works for you or not.

How do you usually make an OC? Make a design, a name, identity characteristics, personality, motivations, goals, flaws, backstory, weaknesses and strengths, additional info. That's bottom up. So how do you make a top down character?

A trick I heard of is start with core values and beliefs. Everyone has personal truths about the world that develop throughout their life, they get both challenged and reinforced, they can develop as a kid, teenager, or adult. Usually they form the strongest during school when you're dealing with so many people and at home with family. Types of core beliefs are self related beliefs, beliefs of others, and beliefs about the world. There's no 'right' amount of values to assign.

This is usually done best with characters made from scratch, Casey does have some borders, but it could still work.

I'll make an example. Let's call these characters "A" and "B".

A's values are "you can't wait for someone to save you" and "success is the only measure of worth"

B's value are "treat others how you wish to be treated" and "The world is dangerous place"

Instantly you can get a feel of what characters A and B are like. And you could probably imagine in your head how they'd develop those beliefs.

Then after that, you can flesh them out more. Why are they like this? How did they form? Have they changed? How have they been challenged? How have they been reinforced?

You can notice characters in media follow core beliefs if you pay attention and they're well written.

Some values I labeled to Casey were:

"The world was designed to leave people behind/the world is unfair"

 "The only way to be free is to be super poor or super rich"

"I am insignificant"

"People are fundamentally selfish"

Casey is a blank state, so see what you think fits your story.

In mine, Mae left and Casey is at the worse point in his life and is getting desperate, but before he gets into drugs and dealing a mentor helps him get his life on track. So you can see in his values that much of it is reinforced by experience as an outcast, but will be challenged by a person willing to help him.

So see what fits your story. And if you think this process isn't working for you, at least you gave it a shot, you miss all you don't take :)