When your a mind fairy you can see things ones mind does not allow, you can have access to thoughts, memories, and dreams but sometimes these thoughts are something you do not want to know about a person. For Darcy Williams being a mind fairy meant feeling what others think of you, that's probably the reason why she stays distant from everyone else including her dad, sometimes it's just too much to take in and most times it just completely makes her unstable. (Rivenxoc season 1-2?)