this article may contain major plot or ending details
- Bertolt – “Thank you Ymir... I'm sorry.”
- Ymir – “It's fine.”
- — Bertolt apologizes to Ymir, Chapter 50
Bertolt and Ymir were both graduates of the 104th South Division and were often seen together with Historia (known as Krista by then) and Reiner, although in the anime. Prior to the Female Titan arc, the two's interactions were minimized. At some point, they were assigned at the same squad, and Ymir questioned Bertolt over where Krista was, however, he was unknowing of her position.
In the Clash of Titans arc, the two are assigned to stay inside Wall Rose in their civil clothes. Nanaba reports to them that Titans have breached Wall Rose, and that they must look for it. Being unsuccessful, they take refuge in Castle Utgard, where Titans attack. One of the Titans get in, and Reiner lets himself get bitten by it. He calmly suggests to jump out the window with it, but Connie takes his knife and cuts it's jaw open. Both Ymir and Bertolt threw it off, and the titan fell down to the ground. Then, they go back to the roof and see that Nanaba and Gelgar have died. Ymir decides to take Connie's knife, runs and jumps off the building slices her hand, and turns into a titan. To much of Bertolt and Reiner's horror, Ymir's titan was the same titan Marcel Galliard, their friend, had.
The Scouts save everybody, and are taken to Wall Rose where Ymir is unconscious. Bertolt and Reiner transform into Titans after their talk with Eren, and capture Ymir. Bertolt carries her onto Reiner's back, and they escape to the Forest of Giant Trees. There, Ymir views Bertolt as an enemy when she wakes up and refers to him as a bastard to Eren.
When it is seen that the Survey Corps have arrived to rescue Eren earlier than planned, Bertolt goes over to Ymir to take her, asking if she remembers eating Marcel, one of his friends. Ymir states that she doesn't remember killing him, and apologizes. Bertolt forgives her and says that it was the same thing with him and his comrades. As they are leaving, Ymir observes that Historia is coming with the Scouts on the front lines. She tells Reiner to get her, but he argues with her that they don't have the time. She then gets in the way of Bertolt's maneuvering, blocking his view with her arm by putting her hand onto his face, making him nearly crash into a tree. She tells them that the environment is perfect for her titan and threatens to eat Reiner and Bertolt and take Eren back to the Scouts. Reiner allows it, and she goes to kidnap Historia and talk to her. After Ymir and Historia talk, and Historia declares that she is on Ymir's side, which shocks Ymir. Bertolt gets worried and asks Ymir is she had switched sides, saying that they've wasted time saving Historia and that the Scouts are catching up.
Later, Connie asks if Bertolt and him are really enemies, and Bertolt claims that he did indeed see them as comrades, and that he didn't want to kill people in the first place. Ymir appears to sympathize with this and understands his actions. When Bertolt is on the verge of death, Ymir saves him and they go to Shiganshina, where they take a rest after running away for two days. Bertolt asks Ymir why she saved him, and Ymir says that she did it because she felt as if she owed a debt to him; indirectly turning her back into a human and breaking the Wall down. Bertolt starts crying and thanks her, telling her that he truly cares about her and didn't want her to die. Ymir retorts by saying that it's fine.
Controversies have surrounded this ship when seen romantically, as Ymir is confirmed to be a lesbian, although a few fans nonetheless ship them as a couple. They do not have many works on AO3 on the gen side. While in terms of friendship, EreYumi and ReiYumi are more popular, fans like their interactions, and their sweet/selfish dynamic.