@Lucastukkp 1.
Your OC could have a simple but noble motivation — for example:
“I don’t want to become a demon, not because I hate them. I just want to prove that humans can save the world.”
This motivation sets them apart from other characters like Issei, who ultimately became a half-dragon-demon. Your OC could become a “hero of the human world” who acts as a bridge between humans and supernatural races.
2.
Show it through interactions and life philosophy:
She's imperfect, can be weak, afraid, and make mistakes—but she still stands up.
She values ordinary life, like eating together, laughing, or falling in love without a supernatural contract.
She might say to Issei or Rias:
"You have incredible power, but you've forgotten what it's like to be human. I don't want to forget that."
Words like that can touch the hearts of other characters.
3.
Emotional strength often comes from small, sincere words.
Issei: “You’re not afraid of death, even though you’re only human?”
OC: “I am. But it’s precisely because I’m afraid that I’ve learned to be brave.”
Dialogue like this can show the essence of being human—fearful, but still daring to move forward.