You can’t have serious trouble in a play without at least a half-serious villain.  Don Pedro’s illegitimate brother John (played here by Abby Schmidt) is a pure bad guy: only happy when others are miserable, miserable all other times, he makes his way across this otherwise footloose landscape sowing slanders and hatching vicious plots that just might work.  Here, he explains himself:

Conrad:
What my lord! why are you thus out of measure sad?
Don John:
There is no measure in the occasion that breeds; therefore the sadness is without limit.

And from a little later in the play, when his henchman Borachio comes up with a plan to prevent the marriage of Hero and Claudio, he is all ears:

Any bar, any cross, any impediment will be medicinable to me.  I am sick in displeasure to him, and whatsoever comes athwart his affection ranges evenly with mine.  How canst thou cross this marriage?