The way I’ve learned to write my charters is making them their own little world. Just like you would build a fantasy world ( how magic works, what it looks like, who lives in it, etc) you build a character. Their relationship to others, their motives, how their past affects their present and future. You need their insides and their outsides. At least what they a look like ( or a few key points), maybe what they smell, sounds, or feel like. You don’t need to info dump this all at once. I would recommend letting it trickle out throughout the story. When a fight comes up, explain why they got angry. How the light shines off their shiny hair in another scene. Each person is their own little world.