Spoiler

Well it's the Ymir/Reiner/Bertolt twist!!! Literally one of the most explosive things in this narrative.

On top of that, the gut-wrenching moral affirmations from Ymir to Krista, on wanting to have Krista live her own life, to live for oneself––how sometimes trying to be "selfless" can be a selfish act, if you look at your motives. That's what's amazing about this series, how it plays with expectations and perceptions and the moral definition of things.