Hayate: a ninja that is excessively fast, will always get a turn before your party, and can hit damage in the quadruple digits with ease, when your characters are more likely to have only 300-700 HP, and their attacks reduce your magic points.
Leaper: a harlequin-looking undead that is also extremely fast, and whose attacks have a chance of causing one-hit kills, and if they don't, they cause enough damage to likely kill you anyway, plus their attacks can drain your levels.
Most Annoying Sound: While incredibly useful, the noise that comes on when the Bishop's Magic Wall is activated becomes incredibly grating to the ears after a while. The characters' Focus unfortunately use the same noise as well.
Running into a wall (which will happen fairly frequently, either by accident or in a dark room) will cause your entire party to whine and complain. Funny the first few times, but gets quite grating afterward.