If we go by the assumption that Mikasa is not Eldian, she’s the only one in the main cast who could just leave and not have to deal with this. She could go and lead a normal life and integrate in the world in a way that no other character can.
But we all know she won’t do that. She would never, because of the love and care inside of her. She didn’t grow up being taught to hate the Eldians, they’re her family and just by existing, just by the bonds she has, she defies Marley and the ideology they push. Marley spreads fear, they deliberately send soldiers traumatised by the Eldian titans back home to make sure the Eldians are feared and hated by everyone; and Mikasa, someone who is not Eldian, who has had her entire life shaped by these people she loves, who she fights for and who fight for her, is every single thing the oppressive system in Marley is trying to prevent.