The last episode of "Attack on Titan" season 2 features Ymir's secret identity as a shapeshifter. Her dearest friend will also do the same in the next episode. The revelation of Krista's identity and possibly other shapeshifting Titans will be featured next episode.
Even before the "Attack on Titan" season two premieres, there as been spoilers from the showrunners about the most focal characters. General Director of the new season Tetsurou Araki has revealed that focal characters include Krista and Ymir, as reported by Comic Book. Their moment to shine will come in the next episode, as a follow-up of the group's trouble at Utgard's Castle.
Krista and Ymir have been inseparable in the last season. In fact, Ymir's constant reminder for Krista to marry her has been one of the comic relief of the show. But Ymir's life is in danger as she protects her comrades against the Titans by shifting into her Titan form. "Attack on Titan" season 2 haven't revealed yet if Ymir is one of the enemies inside the wall or her loyalty remains to Krista. Fans are also dying to know the promise the two has made in the snowy mountain.
Reiner and Ymir's past is connected, as the Titan that ate Reiner's friend is similar to Ymir's Titan form. Krista seems to have a dark past too. It will be shown in the next "Attack on Titan" season two episode what or who Historia is. It is a possibility that Historia is Krista herself.
"Attack on Titan" episode 4 seem to foreshadow some of the key events that will happen in the story. Reverend Nick will protect his church's secret even if it costs his life, as explained by Crunchyroll. Is Historia's secret connected to Reverend Nick's secret? Updates will keep coming so stay tuned.