Decide what is relevant about this person you’re writing about, and how to say it effectively.
Consider both the outer and inner person, demonstrated through actions, reactions, choices. What drives him, what is he proud of. Select what you will reveal carefully.
True character is revealed by the stress he is under and his choices. A protagonist takes action from his point of view, it appears reasonable to him, but it releases the forces of antagonism. Emphasize reactions – the rift between expectation and result – that puts energy into the story.
Dialogue can reveal character better than paragraphs of description. Think about his choice of topic, why he’s angry about it, or sad, or proud. How will he select words to express his feelings and ideas. How will he try to get what he wants by pleading, or arguing.
Show the strength of his need or desire by what he is willing to risk for it. The greater the risk, the stronger the story.
Have your character manouvre himself into a dilemma, then make it worse. Find something that logically but unexpectedly will rip him out of it, but put him in an even nastier situation. Show how he grows, or fails yet again.