It is common for Attack on Titan fans to speculate on the relationships between characters. Look no further than those theorizing about Eren and Mikasa. As such, Ymir and Historia are no exception to this. Their ship name has been unofficially called YumiHisu by fans. Questions sprang up after seeing the two often together: Were Ymir and Historia together? Were they an item?
Their relationship could be considered quite ambiguous, as Hajime Isayama himself never confirmed anything about it. But the producer, George Wada, confirmed that they are actually canon in an interview.

Some fans may not consider it canon because it did not come from Isayama himself, while some will wholeheartedly agree to it. After all, some doubts can still rise because Historia did not express much romantic sentiments towards Ymir. Ymir was the more explicit one.
There are references in the manga and anime that can serve as the guide to determine whether or not Ymir and Historia were together.
They Were Physically Inseparable
In Season 1, it was clear that Ymir and Historia were close. They do everything together. Their relationship can be interpretated as close friendship. But during their battle in Trost, Ymir said, “That’s my Krista! After this, marry me baby!” It can indicate that Ymir feels some sort of attraction towards Historia. Although Historia never responded to an affirmative, we can say that she doesn’t seem to mind it. If she truly did feel uncomfortable, then she would have avoided Ymir afterwards. Or rather, shut Ymir down gently. In fact, they even got closer after the ordeal in Trost.

Ymir Inspired Historia to be Better
In the earlier seasons, Historia has this ‘Krista Lenz’ persona. She was very selfless and strived to be liked by everyone. At first, Ymir may seemed rude to call out on Historia’s seemingly harmless behavior. As the episodes went on, it was revealed that the ‘Krista Lenz’ persona is something detrimental to Historia on the long run. It is not her real identity, and in by doing so, Historia would have to hide under a mask.

Considering that Ymir has a backstory where she has to pretend to be someone else, there is a sense of kinship. They have the deeper connection where they truly understand each other. Even when Historia did not like Ymir’s bluntness, it is what she needed to overcome her fear of being herself.
Ymir said something that left a deep impact on Historia: “Krista… I have no right to tell you how to live. So this is just… a wish of mine. Live your life with pride.”
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This is the turning point for Historia to start thinking for herself. She was able to reveal to Ymir her real name, which meant that their relationship got even stronger. It can even be labeled as ‘intimate’ now.
It was Ymir’s influence that pushed Historia to become a better version of herself. It was why she had the courage to defy her father in Season 3. She became more sure and confident of herself, which earned her the rightful place in the throne.

Protecting Each Other
It was made very apparent how deeply Ymir and Historia cared for one another. Ymir, in particular, has only one goal and that was to protect Historia. It was the reason why she joined the Survey Corps. Originally, her intention during military training was to slack off so that Historia can be on the Top 10. In this way, Historia will get the chance to join the Military Police – which is a safer branch located inside the Walls.
Ymir even transformed as a Titan so that Historia and the others were safe.

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Historia, notably, felt very anxious whenever Ymir was risking her life. This happened when Ymir first transformed into a Titan. Historia was the very first person to come to Ymir’s side when the latter got severely injured. The former was almost in tears when she thought that Ymir was dead.

She was also very anxious when Rainer and Bertholdt have kidnapped Ymir and Eren. She was the one very much concerned for Ymir when everyone else talked about Eren.
Historia’s Devotion
During the whole escapade of Rainer, Bertholdt, and Ymir teaming up against Eren and the others – Historia seemed confused about what Ymir was planning to do. After all, the three Titan shifters from Marley were quite vague about their plans. Ymir seemed intent on getting Historia with her back to Marley, where she believed it was safer.

Historia, not understanding the plan, still trusted Ymir. She said these lines: “No matter what happens… I will always be your ally!” This alone indicated how strong her feelings were towards Ymir. It’s a sign of being there for each other despite all odds.
This tied strongly towards Historia’s feelings of comfort towards Ymir. She admitted that she does not feel any fear when she’s with Ymir. She knew she can freely be herself. It was almost as if she considered Ymir as a home where she just felt safe.
Historia’s Breakdown
When Ymir left Historia to return to Marley with Rainer and Bertholdt, Historia felt devastated. Her breakdown was not explicitly shown in the anime. In the manga, however, she expressed how heartbroken she was.

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It was clear that Historia was looking forward to be with Ymir for a longer time. Now that Ymir has left her, she was left into pieces. It indicated that Historia’s feelings were deep.
On top of this, Historia seemingly felt empty. She does not have the usual vigor in her, nor the Krista Lenz persona. She almost fell prey to her father’s lies, and contemplated on injecting the Titan serum in her. However, it was Ymir’s words who grounded her back into reality. To live a life she was proud of – and she definitely did not want to become a Founding Titan like her sister Frieda. Carrying a heavy responsibility that costs her own humanity.

Despite her grief, Historia was able to bounce back. And it was all because of Ymir’s impact on her.
Ymir’s Farewell Letter
This is probably what solidified Ymir and Historia’s relationship. Ymir wrote to Historia that she has no regrets. The only regret she has: she was not able to marry Historia.

This was not the first time she offered marriage to Historia. Although the first one can be interpreted as something light or just flirty, this time is different. The letter was solemn, and intended to be the last goodbye. This meant that even back then, her comment was actually serious. Historia, instead of getting weirded out, got emotional instead.

This alone implies that Historia might have felt the same way. She also regretted that she was not able to marry Ymir. It is important to remember that Historia wanted to live her own life with Ymir together.
It would be strange for Ymir to talk about marriage if Historia had not felt the same way. If Historia never gave any indication that she also liked Ymir romantically, then Ymir would not have mentioned marriage. She probably would have went along the lines of: “I only regret that I never got you to love me“. Generally, it should have been stated in the letter if Historia never reciprocated those feelings.
George Wada Was Right
His claim actually has solid arguments to it when referencing the anime and manga. It was clear that Ymir and Historia both harbored strong and deep feelings for each other. Even though Historia never showed it, Ymir is someone she holds in her heart. Their relationship is considered intimate and their connection is felt. It is not far-off to consider their relationship as romantic.
Although Historia did end up with someone else, Ymir became an important part of her life. Historia will always remember her, even in the years to pass.

Some fans will stubbornly insist that friendships can be intimate without being romantic. Well, to each their own, as anyone can interpret the characters and relationships of their favorite anime freely. In consideration of the producer’s claim and the references in the anime and manga, though, it is safe in saying that Ymir and Historia were together.