How do you really know Ferris is any dere? From what I can see, she hasn't shown any romantic interest whatsoever in Ryner. She only showed compassion during that one scene when she saved his sanity, but that's it. Other times she's always beating or insulting him, at best using him as a slave. Such things would be very common in -dere behavior if it was actually in combination to somehow indicating she cares for him, but I haven't seen a single sign of that being the case.
Now, I'm a person who would like most shows to have some romance in addition to the main theme and thus I'm certainly not biased against it, but sometimes it just isn't there.