In an interview, Hajime Isayama, creator of "Attack on Titan," talked about the future of several protagonists in the manga, including Ymir. He also explained how the characters are going to be developed in the further story.
According to Yibada, Isayama hinted that the key to completely understanding the entire manga series lies with Ymir and she will lead the story to "ultimate conclusion."
In the magazine "Gekkan Shingeki no Kyojin," the creator of one of the most popular manga series of all times revealed that initially he had planned Sasha Blouse for playing the role of Ymir.
However, afterwards he decided to create an entirely new character who is now a fan favourite for her comical and humorous gestures.
"Compared to other characters, Sasha's profile was the last one to be finalised. Her physical appearance details and her fundamental image were already planned at initial stage, but at that time her name was Ymir," Isayama said.
Isayama explained that he wanted the character of Ymir to be mysterious and dark as she possesses a secret that will define the story and will also lead to the conclusion of the manga series.
It seems that Ymir's character was intentionally kept under dark for most of Season 1. The only significant scene she had in first season was when she was involved with a hungry Sasha and Krista Lenz at the barracks.
Krista will also undergo a major change as, in the upcoming season, she will gradually get out of her cute persona and will become more bold and tough.
Though Isayama didn't reveal much about what secret Ymir holds, it seems that it must be something about the origins of the Titans and the world from where they came.
Ymir hasn't appeared in the manga since "Clash of the Titans" story arc. Some fans speculated that she was used as a serum sacrifice for the warriors, but after the statement of Isayama, it seems she will have a significant role in the story.