I don't design my characters around a core motivation, but typically one will reveal itself as I get to know the character more. I also try to distinguish between a character's conscious motivation (want), and what they actually need, which drives them subconsciously.
My main character wants to chill in space, do his job, and tinker/experiment with rune spells. He knows he's not happy, but he's content enough.
What he needs is purpose, place, and people. All things which he's lost before, and which he fears losing again.
And what drives him to take the actions he takes in the story is the impulse and obligation he feels to pitch in and do what's right when he's faced with something unjust.