Shingeki no Kyojin creator Hajime Isayama dropped several hints in an interview about the future character development of several protagonists in the story, including Ymir who apparently holds the key to fully understanding the entire manga series.
In the magazine issue entitled Gekkan Shingeki no Kyojin, the man behind the worldwide anime hit Attack on Titan revealed that he originally planned Sasha Blouse to assume the role of Ymir, but decided to create a new one for the character who has become a fan favorite for being gawky and comical.
"Compared to other characters, Sasha's profile was the last one to be finalized. Her physical appearance details and her fundamental image were already planned at initial stage, but at that time her name was "Ymir."
As for Ymir, Isayama explained that he wanted the character to be shrouded in mystery because the character possesses a secret that will define the flow of the story in the Shingeki no Kyojin realm.
"The current Ymir's characteristics - as someone who gives useful advice to Christa, and as the bearer of a secret that will be revealed as the story progresses - were initially planned for Sasha's role."
That explains why Ymir's character had limited exposure in Season 1. The only major scene she was involved at was with a hungry Sasha and Krista Lenz at the barracks. Speaking of Krista, she will also have a major character development in the upcoming season, gradually abandoning his cute-type persona.
Regarding the secret Ymir holds, everybody is clueless. My hyphotesis is it's all about the true nature of the titan and the history of the world.
Then again, no one knows. Heck, if you are following the manga, Ymir hasn't appeared yet since the end of the Clash of the Titans arc. Some speculated that she was already used as a serum sacrifice for the warriors, although I really doubt it considering how much pivotal her character in the story.