I was just watching a writing advice video by Jed Hurne (love his videos!!!) and he made a really eye-opening suggestion.
Your character should have "four selves."
I thought about this, not only with my novels "Daughters of Ashby" and "Dodatrad Heiress," but the epic WWII story I'm working on, "Dangerous Territory."
Delaney's social self is that of a stern heiress of her clan, firm in posture, talented in weaponry, and well-taught in social decorum. She's a little rough around the edges, but she is one of the most well-rounded heiress in the ballroom.
Her personal self is far more peaceful. She hates the idea of war and would rather sit around playing with rabbits or picking berries than practicing her sword.
Her core self is a struggling young woman who does not feel like she belongs in that warmongering clan. She knows she does not have what it takes to follow in Queen Eilat's shoes, and she feels like being sheltered most of her life has limited her.
Her hidden self craves love and attention, something she definitely did NOT get with Queen Eilat and King Garnock. All she wants is to be a teenager, but even that is denied from her, so when she does make friends, she holds on to them with a sense of desperation.
Eren's social self is that of a young but skilled officer knowledgeable in battle tactics, the ideal German Nazi soldier.
His personal self is that of a soft man who deeply cares for his men and frequently disagrees with orders, but feels trapped to obey them.
His core self is a deeply-in-the-closet man. He struggles with a sense of identity, fears the dangers of falling in love, especially since coming out as gay in Nazi Germany is a death sentence.
His hidden self is a fragile young man, deeply scarred by his past and the loss of his mother and apparent betrayal of his father, already suffering PTSD which just gets worse as the story goes.
Levi's social self is a stoic man with many odd skills. His height puts strangers off their guard around him, something he can use to his advantage.
His personal self is just as stoic, but he can have a wicked wittiness about him. The more he insults you, the more he likes you. He struggles with emotions, but when he does show them, they shine with genuineness. As much as he scoffs at religious people, he goes out of his way to help other Jews and is learning about his heritage along the way.
His core self is a man who struggles with past, with his future, has suffered loss after loss, a lifetime of trauma, and he is long past PTSD.
His hidden self is tormented. His dreams are full of the blood. The lives he took and the lives he could not save haunt him late at night. He tries to live by the motto "No Regrets," but the reality is he is drowning in regrets.