The official website confirms that Ymir's feelings for Krista are romantic in nature (NO REALLY?!).Since then, Ymir's feelings for Krista was a plot point. Anime Producer George Wada also stated at the AniMagic 2014 series panel that Krista and Ymir are indeed a couple. (I can hear Seiko screaming HAHA! FVCK YOU REINER in the distance.)