As one of the main characters of Attack on Titan, Mikasa Ackerman has shifted and changed through every single arc of the anime. She has come far from the little girl who lost her parents, but maybe that isn't exactly where Mikasa wants to be. Despite being the best one there, Mikasa had no desire to be a soldier. When Mikasa joined the cadets, she only did it because Eren and Armin were going.
Mikasa had been in love with Eren since he saved her from human traffikers as a child, and it seemed for the longest time that her only defining character trait was her obsession with him. But Mikasa is at her core a very sweet girl who only wants to stay with those she loves more than anything in the world, whether that's in the cadet barracks, a farm on a hillside, or in a zeppelin flying over Marley. Mikasa's protectiveness of Eren seemed justified in the earlier seasons simply because Eren was the king of talking big and not being able to back it up. When the person you're in love with is constantly getting kidnaped or almost killed, are you actually being overbearing when it comes to protecting them? Mikasa certainly didn't think so.
Young Mikasa with her Mother
Mikasa can get so wrapped up in other people that when she thought Eren had been eaten by a titan in season one, she was surprised by her desire to keep living. Learning that she could live without Eren didn't bring her as much comfort as she wanted, however, because when Reiner and Bertholdt kidnaped him, Mikasa now had reserved knowledge that she just couldn't have what she wanted. She told Armin that the only thing in this world she desired was to be close to Eren, but it seems that's just not possible. Mikasa is aware that this world is cruel, but she also believes that it's beautiful, and that's only because of Eren.
At the end of season three, when Eren explains the curse of Ymir, Mikasa takes it the hardest. Not only is Eren, the love of her life, going to die in about eight years, but Armin, her only other family member, also has a thirteen-year expiration date. Everything she cared about, she was inevitably going to lose. She lost weight and stressed over this realization and seemed to resolve herself to treasuring all the time she has left with her boys. Which is why the shift Mikasa has to go through in season four is so hard for her. When we open in season four, we have gone through a classic anime time skip. The characters are now four years older, and their relationships are completely strained concerning Eren. When Mikasa comes and helps him fight the Warhammer Titan, she looks horrified by his actions in Liberio, and when the Levi kicked Eren in the face, Mikasa allowed Armin to stop her - the complete opposite to how she would have acted in season one.
The distance between Eren and Mikasa has grown the older they've gotten, but only because Eren keeps her at a distance. There was only one time when Eren, in a moment of sullen desperation, asked Mikasa what he meant to her. Mikasa, completely flustered and only a little tipsy, called him family, much to his disappointment. It's after this that Eren commits fully to the Rumbling, and he tells Mikasa he has always hated her.
Being sat down and told that she has no free will and only cares about Eren because she's an Ackerman was one of the worst things Eren could have ever said to her. Mikasa's goals had never been to save humanity, to save Eldia. She only cared about her friends and family, and hearing this while still reeling from Sasha's death broke her heart. Mikasa wants so badly to believe that Eren isn't doing something evil, she wants to believe that he's still the same person who wrapped her scarf around her, but with every atrocity it becomes harder and harder to hold onto that hope, and Mikasa starts to become disgusted with herself for wanting to defend him so much.
Mikasa fighting the Jaegerists
She reluctantly sets her scarf aside, her heart completely shattered, and resolves to let go of Eren if that's what he wants so bad. After the Rumbling began, she and Jean were left in Sina to pick up the debris and deal with the Jaegerists. She goes and retrieves her scarf from a dying soldier who looks up to her, proving that Mikasa doesn't want to let this go. She does not want to condone Eren's actions, and, in a way, wants to save him from himself as usual. When Hange asks for her and Jean's help, she readily agrees, saying with certainty that Eren needs to be stopped.
It is after they join up with the Marleyians and form their alliance that Annie asks Mikasa the most important question: if it comes down to it, could she kill Eren? Mikasa is certain it won't come to that, but as things progress, she becomes less and less sure. The Founder Ymir wants to know what choice Mikasa will make because it will affect how this story ends. If Mikasa can find the strength to kill Eren, the person she loves most in the world, than Ymir can find the strength to stop following the orders of the Eldian king. Mikasa has to pick her own ideals over Eren, and it is the hardest decision that can be asked of her. Because while this world is cruel, it is beautiful, but sometimes in order to do the right thing you have to sacrifice that beauty.