- “No matter what happens... I will always be your ally!”
- — Historia to Ymir
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Before joining the military, Ymir lived in Wall Sina as a spy and thief, one day she overheard a conversation by the Church of The Walls and heard them speak about Krista. Eventually, Ymir began to gain interest in the girl named “Krista” which led her to join the 104th Training Corps as a cadet.
On the first day, Krista is spotted by Ymir as she sneaks out of the dining hall to give food and water to another classmate, named Sasha. Sasha was punished by the instructor and was told to run until she dropped. Ymir then approaches the two and asks Krista what were they doing, but Krista tells Ymir that she wanted to help Sasha until the brunette girl collapses onto Krista’s lap. Ymir and Krista both help Sasha into her sleeping bed which afterward Ymir tells Krista that she could take advantage of Sasha due to her being an idiot. Sometime later Ymir confronts Sasha for being too polite and tells her to be herself until she begins to criticize Sasha until Krista jumps in to defend Sasha. Krista begins to tell Ymir to leave Sasha alone and that she can be who she wants to be.
During their training, the two and the rest of the cadets have finished their training in the snowstorm, however, a cadet named Daz loses his conscious and Krista volunteered to pull the unconscious cadet, Ymir is shown to telling to give up on saving Daz after. As Krista listens to Ymir’s argument, she tells Ymir that it’ll be best if she went ahead which can mean the survival of the three of them. Ymir begins to realize that Krista doesn’t care about saving Daz but instead is using their current situation as an excuse to die. Ymir begins to tell Krista that she knows who she is and by telling her that she knows she’s the King’s misteress’s daughter, she also says that she knows that by overhearing the conversation of the members of the Church of The Walls and tells Krista not to worry as she won’t spill out the information. Krista then asks Ymir if she joined the 104th Training Corps just to find her and why if she did. Ymir tells Krista that she joined because she felt some sort of connection with her, but then Krista tells Ymir if it was also because she wanted to befriend her until Ymir tells her it isn’t like that and that they’re nothing alike. Ymir explains to Krista that it was most likely due to having a second chance at life but at the same time she could never use a fake name like Krista was using at that time as it’ll defeat the purpose of avoiding defeat. She further explains to Krista by keeping her name she’ll and live her new life to get revenge just by staying alive, Ymir then scolds Krista for wanting to commit suicide and instead of wanting to kill her enemies.
Krista then realizes there is no way for them to survive until Ymir brings Krista to a cliff where the camp can be seen. Ymir brings up the idea of throwing Daz off the cliff so he can be found later and so the two of them can head to the camp with no problem, but Krista disagrees and thinks that Daz won’t survive the fall from a high cliff. Ymir tells Krista to shut up and tells her to get back to the camp herself as she’ll go down the cliff before throwing Krista into a tree, making a pile of snow falls on top of her, as Krista gets out from the pile of snow Ymir and Daz are nowhere to be seen.
Krista heads back to the camp and finds Ymir, as they talk Krista notices the height of the cliff where they were and questions Ymir how did she do it and how did she keep Daz alive from such a height. Ymir then begins to tell Krista that’ll she share how she was able to save Daz but in one condition, that’s if Krista will promise her to take back her old name and live. At present time Ymir once reminds her about their promise before jumping off Utgard Castle to fight off the titans in her Jaw Titan form as the titans were trying to eat Krista, Reiner, Bertolt, and Connie as their superiors had died from being decapitated and later eaten. Christa watches Ymir transform into her titan form and is seen to be visibly shocked. Ymir begins to get attacked by the titans until Christa begins to yell at Ymir to not die and not to be stupid to die for someone else as she should live for herself. She continues and tells her if she’s trying to die while protecting the tower then she should just let it fall. Ymir listens to Historia and begins to use the stone bricks of the castle before heading to Christa and the rest on the top telling them to get on her to live. As the castle begins to fall it Ymir takes them to a safe place from falling off the falling castle. The castle falls on top of the Titans but doesn’t manage to kill them, Ymir is then attacked by the hoard of titans and while she is being attacked Krista then runs to Ymir telling her that she still hasn’t told her real name yet to her. A titan appears in front of Krista and before being nearly eaten she is saved by Mikasa and the rest of the Survey Corps. As Ymir is saved from the scouts, Krista can be seen cradling Ymir who was heavily injured. Ymir begins to wake up and Krista then reveals her real name to Ymir, Historia Reiss.
Ymir is later taken by the Survey Corps due to being heavily injured, as Ymir is resting Historia and Hange are talking, Historia is telling Hange to believe her as Ymir fought in her titan form to save those in the castle. But then Historia further explains to them that she knows what Ymir did was wrong for holding useful information but at the same time says it was also most likely that Ymir wanted to protect her comrades and is on the side of humanity. Hange then agrees with Historia but tells her they hope that Ymir will most likely corporate with them. Moments later, Reiner and Bertolt reveal themselves as the Armored and Colossal Titan, as they transform into their Titan forms they kidnap Ymir, along with Eren. Historia is seen to be horrified and even yells out Ymir.
- Historia – “I'm so glad, really. You had me so worried...”
- Ymir – “I'm sorry, I made you worry. But I told you to believe in me, didn't I?”
- Historia – “I believed, I believed... but...”
- Ymir – “Hey, don't cry Krista. I came back as promised.”
- Historia – “Ymir!”
- Ymir – “Haha, to cling onto me like this is so unlike you. I'm a bit happy.”
- Historia – “Saying something weird at a time like this... you really are the Ymir I know!”
- Ymir – “Haha, isn't it obvious? Don't be silly.”
- — Historia worries over Ymir, Attack on Titan: Escape from Certain Death
- “Well I can understand feeling awkward after seeing our passionate love, but well you are an exception.”
- — Ymir about hers and Historia's love to the player, Attack on Titan: Escape from Certain Death
It's Ymir's official theme song, the song is played during Ymir's flashbacks when she became a mindless titan, then ate the Jaw-Titan holder, and her view on Historia. Ymir is shown to be first learning about Historia during back when she was a thief and spy. More into the flashback Ymir then realizes how she and Historia are similar, lying to themselves.
It's the official theme used for Historia. The song is played when Historia has to make a complicated decision to insert the syringe into her body that is filled with titan fluids in order to eat Eren and gain the powers of the Founding Titan. Historia then disobeys her father as she gets memories of Ymir. The flashback heads back to Utgard Castle before Ymir fights off the titans. Ymir tells Historia that she has no right to tell her how should she live her life, and to instead that Historia should live a life she's proud of. Ymir's words cause Historia to throw the syringe, and to reject becoming the Founding Titan. In one of the song lyrics it mentions how two people kissed, most likely implying Historia and Ymir have had shared a kiss before.
YumiHisu is one of the most popular ships in the Attack on Titan fandom. It's also the most popular femslash ship. Fans were drawn to it given the two's loyalty to each other, as well as Ymir's flirting. The ship being confirmed "Word of God" made many YumiHisu shippers very happy.
On AO3, YumiHisu is the most written ship for both Historia and Ymir. It is also the fourth most written relationship in the Attack on Titan tag.
After confirmation, some non-fans of this ship didn't approve of the confirmation and believed it was "one-sided" on Ymir's side, or that just because a director confirmed the relationship between the two didn't mean it was canon, as they believed it should be Isayama confirming the two, but it's known that Isayama works with the directors when the producing the anime on his wants and removals (ex: Levi choking Historia due to her thoughts on becoming Queen scene was removed in the anime by Isayama's request), especially when it comes to deadlines. However YumiHisu fans argued that even before their ship being canon it was in fact true that the two were in love before developing an official relationship, as the Attack on Titan Guidebook confirmed that the two felt some sort of attraction towards one another before Season 2. Confirming even more of this ship's canonical status.
YumiHisu fans applaud more for this ship as even official merchandise confirmed their feelings for one another (upper left of circle in Japanese). In Historia's card, her highest stat was "feelings for Ymir", meanwhile in Ymir's it states the same, "feelings for Christa (Historia)". Most animators from WIT and MAPPA Studio drew the two, one where Historia is holding Ymir. While the other Ymir is carrying a joyful Historia around the ruins of Utgard Castle.
- It's one out two of Historia's canon ships, the other one being FarmHisu.
- Isayama has stated before that he only drew freckles on Ymir so she would look like a commoner, looking different from Historia. 
- Ymir and Historia were nominated for Best Couple in the 2020 Crunchyroll Anime Awards. 
- "Furthermore, he (Attack on Titan producer George Wada) confirmed the characters Christa and Ymir to actually be a couple and expressed that he found any kind of Doujinshi for the show 'very interesting.'"
- Attack on Titan Character Encyclopedia, (p.36)
- Best Couple 2020 Crunchyroll Anime Awards Nomination