It would be a mistake to discount the thematic value of Ymir in Attack on Titan. Not Ymir Fritz, Ymir, the nameless girl off the streets of Marley turned queen of Eldia. Even though she plays a noticeably small role in the grand scheme of the story, I can proudly say that her character is one of my favourites in the show, despite the wells of depth exhibited by every single one of them.
A radical group of Eldians picked a kid who belonged to no name and named her after their all-powerful founder, Ymir Fritz. At first, this kid had nothing, but all of a sudden, these people would sacrifice what little they had to alleviate any discomforts she might face, given the condition that she bring them even the smallest hope of freedom.
Understandably, this kid made a mistake. She assumed that, for these folks, what mattered was who she was, that she was the one too precious to be seen as dispensable. She didn’t realise that all she was was her royal blood, the woman who can command titans once again and restore their empire. Beyond that, she was still no one.
When Marley finally discovered this group of Eldians, Ymir was immediately blamed for her lies, that she used her royal blood to control and manipulate these innocent folks. When it came down to the moment of truth, she chose to fall on the sword and save her people, taking this blame upon herself.
The second mistake she made was assuming fate as a concept; everything that happens has a cause and effect carefully crafted by some guiding hand—that her fate would be fulfilled here if she becomes a martyr to her people. Instead, the entire cult, even the supposed victims of the devil-reincarnate Queen Ymir, was sent to Paradise. At the edge of the island, she was treated to the merits of her sacrifice—what it all amounted to.
In another moment of—What a younger Ymir would've called fate, she was able to trade away her death in exchange for another's life. At the Path, with absolutely no possessions, no followers, no name, in a desert holding the real Ymir—a slave even in death, this woman only had this extremely laughable set of circumstances in her mind. There was no fate written, she realised after all this time.
“If there was such a thing as fate, I couldn't help but laugh at how fickle it was.”
There was no karmic retribution, no greater destiny, nothing at play here. Her previous life had devoid her of any choice at all, everything was an externality. Even the one decision she made—the decision to call save her people led to the exact same outcome. But now, she was in Paradise. Everything was hers. Every decision she makes will be for her sake, and no one else's.
She, of course, chose to use this free will to be a thief. She was not Ymir Fritz, she was a powerless nobody. She had no responsibility to anyone in the world, not even to herself. Her life was hers to do as she pleased. Nothing was sacred, and everything she could get her hands on was hers.
This routine went on for her until she overheard two men discussing the illegitimate child of a nobleman who was to be shipped to her death at the frontline. They talked about this poor kid being stripped of her name who would have been better off never being born. They decided her life and her freedom as something that can be given or taken away.
Familiar beats of a rotten fate echoed in Ymir's mind. She heard of another young girl as a sacrifice for the greater good. She gets to die a hero, and they get to live.
As she had learned, the only action needed to rebel against a world that doesn't want you was to stay alive. She saw a chance to right the wrongs her younger-self made.
Christa Lenz was, however, difficult to break out of her mindset. She would steal food—not for herself, but for the potato girl whose suffering didn't affect Crista at all. Even as she tried to exhibit the rule of self, hoping to use Sasha in the future instead of serving in her best interest, Christa failed to understand Ymir’s reasoning. She would drag a nearly dead person to refuge in hopes that her legacy would amount to that of a hero, she would join the suicidal maniac despite the ability to live comfortably within the walls of corruption, yet still cry at the prospect of facing death—A reluctant hero pretending to be as virtuous as her fellow soldier.
Ymir however, found herself to have strayed from her beliefs. Why did she join the scouts too? If her belief was still as strict, she would've decided Christa to be beyond hope and continued to live out the rest of her life for herself. However, she found someone more important than herself—Not so she could die a hero, but so she could make sure this one person lives a long, happy life—an option she never had.
There was something about an innocent drive to be self-sacrificial that really annoyed Ymir. At this point, her actions were less determined by her own desires and more by Christa’s decisions. Once again, factors outside her control are toying with her life. This time, however, she accepted them.
When she chose to sacrifice herself to save her comrades—no, to save Christa, for a brief moment, she decided that her will is still her own, that her desire to save the woman she loves is determined by nothing but her own feelings. No one dare declare her a hero.
Within this crazy span of two-three days, she also learned of her other option—to hand herself to Reiner in exchange for Historia's safety. If she agrees to return the Jaw to Marley, she will get to spend her last days knowing Historia will be alive in that country. This life WILL come at the cost of her freedom, though, as she will have to spend the remainder of her days making royal-blooded children for Marley.
But Ymir’s priorities still lay with herself. Her desire to see Historia again and be with her for whatever time she had outweighed Historia’s freedom. She was well within bounds to act selfishly once more. She probably wasn’t, but it doesn’t matter, even if it means looking her in the eye and lying to her face, letting Historia think that her compliance will be a heroic act to save Ymir.
Historia, however, had the realisation she was due from the start. She was tired of Ymir showing the fickle-mindedness she used to condemn, where she became a victim of her feelings for Historia and stopped living for herself. Reiner, Marley… they could not decide what will happen to Ymir and Historia.
In the end, she saw Eren was the coordinate. She realised that, in truth, the power was within the walls, she was able to make her final decision on her own terms. After stealing for years, she just wanted to give back what she owed, even if it means a painful goodbye. No part of this was a decision forced on her by Reiner, by Paradis, or even by Historia. Ymir decided Ymir’s fate. She never did give up her feelings for Historia, though. Those remained till her very last moments—with a single declaration to a hateful world that her emotions were entirely romantic.
I hope this analysis helped you see why I believe Ymir was the thematic backbone for one of the best seasons of this amazing show, especially after understanding Eren’s arc. The delicate balance between the desire for freedom, for love and the sacrifices associated with morality, is heavily influenced by one’s circumstances and nature.