Ymir has evolved into one of the most pivotal characters in the "Shingeki no Kyojin" manga series but in the "Attack on Titan" anime adaptation, very little time has been spent on her. This is all about to change as series creator Hajime Isayama revealed that season 2 will shed some light on her story.
Speaking in an issue of Gekkan Shingeki no Kyojin (via Yibada), Isayama stated that Ymir holds the key to the "ultimate conclusion" of the whole series and that the overall plot of "Attack on Titans" as well as the history of the giant Titans relied heavily on Ymir's story progression.
It is noted by many fans that the name Ymir may be based on Norse mythology. Ymir was the first being to come into existence and is both the father of the Nordic giants and later the gods such as Odin. When Ymir was killed, his body was used to construct the cosmos.
This hints that Ymir may be related to the origins of the Titans or at least how they began to spread across the world, eventually leading to the dwindling human population. Fans will recall that Ymir spent roughly 60 years as a Titan before she regained her human form, and the events prior have not yet been explored in the anime series.
In the manga, Ymir has not made an appearance following the "Clash of Titans" story arc and it was presumed she was dead, but Isayama's statements suggest otherwise. It is possible that she will appear again in the future as the origins of the Titans continue to be explored in the manga and anime series together.
The character did not hold much importance in season 1 and this explanation now sheds some light on this. Ymir may have been kept in the dark on purpose, to allow her character to get the spotlight she deserves when season 2 launches later this year.
