Board Thread:Questions and Answers/@comment-29752445-20170527165653/@comment-5690373-20170527201123

I suspect Reiner expected her to know that because the titans and their abilities are probably famous in Marley, but he didn't know Ymir had been trapped as a mindless titan for 60 years so she couldn't have known about the Beast titan's ability to turn people into titans that walk at night (the ability that Zeke discovered because of his royal blood).